commit 3c3b04d1 upstream.

Due to insufficient check in btrfs_is_valid_xattr, this unexpectedly
works:

 $ touch file
 $ setfattr -n user. -v 1 file
 $ getfattr -d file
user.="1"

ie. the missing attribute name after the namespace.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=94291Reported-by: William Douglas <william.douglas@intel.com>
Signed-off-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Chris Mason <clm@fb.com>
[bwh: Backported to 3.2: XATTR_BTRFS_PREFIX is not supported]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>