`fs_kfuncs.c`'s `test_xattr` would fail the test even when the
filesystem did not support xattr, for instance when /tmp is mounted as
tmpfs.

This change checks errno when setxattr fail. If the failure is due to
the operation being unsupported, we will skip the test (just like we
would if verity was not enabled on the FS.

Before the change, fs_kfuncs test would fail in test_axattr:

 $ vmtest -k $(make -s image_name) './tools/testing/selftests/bpf/test_progs -a fs_kfuncs'
 => bzImage
 ===> Booting
 [    0.000000] rcu:        RCU restricting CPUs from NR_CPUS=128 to
 nr_cpu_
 ===> Setting up VM
 ===> Running command
 [    4.157491] bpf_testmod: loading out-of-tree module taints kernel.
 [    4.161515] bpf_testmod: module verification failed: signature and/or
 required key missing - tainting kernel
 test_xattr:PASS:create_file 0 nsec
 test_xattr:FAIL:setxattr unexpected error: -1 (errno 95)
 #90/1    fs_kfuncs/xattr:FAIL
 #90/2    fs_kfuncs/fsverity:SKIP
 #90      fs_kfuncs:FAIL

 All error logs:
 test_xattr:PASS:create_file 0 nsec
 test_xattr:FAIL:setxattr unexpected error: -1 (errno 95)
 #90/1    fs_kfuncs/xattr:FAIL
 #90      fs_kfuncs:FAIL

 Summary: 0/0 PASSED, 1 SKIPPED, 1 FAILED

Test plan:

  $ touch tmpfs_file && truncate -s 1G tmpfs_file && mkfs.ext4 tmpfs_file
  # /tmp mounted as tmpfs
  $ vmtest -k $(make -s image_name) './tools/testing/selftests/bpf/test_progs -a fs_kfuncs'
  => bzImage
  ===> Booting
  ===> Setting up VM
  ===> Running command
  WARNING! Selftests relying on bpf_testmod.ko will be skipped.
  Can't find bpf_testmod.ko kernel module: -2
  #90/1    fs_kfuncs/xattr:SKIP
  #90/2    fs_kfuncs/fsverity:SKIP
  #90      fs_kfuncs:SKIP
  Summary: 1/0 PASSED, 2 SKIPPED, 0 FAILED
  # /tmp mounted as ext4 with xattr enabled but not verity
  $ vmtest -k $(make -s image_name) 'mount -o loop tmpfs_file /tmp && \
    /tools/testing/selftests/bpf/test_progs -a fs_kfuncs'
  => bzImage
  ===> Booting
  ===> Setting up VM
  ===> Running command
  [    4.067071] loop0: detected capacity change from 0 to 2097152
  [    4.191882] EXT4-fs (loop0): mounted filesystem
  407ffa36-4553-4c8c-8c78-134443630f69 r/w with ordered data mode. Quota
  mode: none.
  WARNING! Selftests relying on bpf_testmod.ko will be skipped.
  Can't find bpf_testmod.ko kernel module: -2
  #90/1    fs_kfuncs/xattr:OK
  #90/2    fs_kfuncs/fsverity:SKIP
  #90      fs_kfuncs:OK (SKIP: 1/2)
  Summary: 1/1 PASSED, 1 SKIPPED, 0 FAILED
  $ tune2fs -O verity tmpfs_file
  # /tmp as ext4 with both xattr and verity enabled
  $ vmtest -k $(make -s image_name) 'mount -o loop tmpfs_file /tmp && \
    ./tools/testing/selftests/bpf/test_progs -a fs_kfuncs'
  => bzImage
  ===> Booting
  ===> Setting up VM
  ===> Running command
  [    4.291434] loop0: detected capacity change from 0 to 2097152
  [    4.460828] EXT4-fs (loop0): recovery complete
  [    4.468631] EXT4-fs (loop0): mounted filesystem
  7b4a7b7f-c442-4b06-9ede-254e63cceb52 r/w with ordered data mode. Quota
  mode: none.
  [    4.988074] fs-verity: sha256 using implementation "sha256-generic"
  WARNING! Selftests relying on bpf_testmod.ko will be skipped.
  Can't find bpf_testmod.ko kernel module: -2
  #90/1    fs_kfuncs/xattr:OK
  #90/2    fs_kfuncs/fsverity:OK
  #90      fs_kfuncs:OK
  Summary: 1/2 PASSED, 0 SKIPPED, 0 FAILED

Fixes: 341f06fd ("selftests/bpf: Add tests for filesystem kfuncs")
Signed-off-by: Manu Bretelle <chantr4@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20231211180733.763025-1-chantr4@gmail.com