reproducer:
mkfs.btrfs -f /dev/sdb &&\
mount /dev/sdb /btrfs &&\
btrfs dev add -f /dev/sdc /btrfs &&\
umount /btrfs &&\
wipefs -a /dev/sdc &&\
mount -o degraded /dev/sdb /btrfs
//above mount fails so try with RO
mount -o degraded,ro /dev/sdb /btrfs

------
sysfs: cannot create duplicate filename '/fs/btrfs/3f48c79e-5ed0-4e87-b189-86e749e503f4'
::

dump_stack+0x49/0x5e
warn_slowpath_common+0x87/0xb0
warn_slowpath_fmt+0x41/0x50
strlcat+0x69/0x80
sysfs_warn_dup+0x87/0xa0
sysfs_add_one+0x40/0x50
create_dir+0x76/0xc0
sysfs_create_dir_ns+0x7a/0xc0
kobject_add_internal+0xad/0x220
kobject_add_varg+0x38/0x60
kobject_init_and_add+0x53/0x70
mutex_lock+0x11/0x40
__free_pages+0x25/0x30
free_pages+0x41/0x50
selinux_sb_copy_data+0x14e/0x1e0
mount_fs+0x3e/0x1a0
vfs_kern_mount+0x71/0x120
do_mount+0x3f7/0x980
SyS_mount+0x8b/0xe0
system_call_fastpath+0x16/0x1b
------

further 'modprobe -r btrfs' fails as well
Signed-off-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Chris Mason <clm@fb.com>