client: allow xattr caps in inject_release_failure

Because some test environments generate spurious
rmxattr operations, allow the client to release
'X' caps.  Allows xattr operations to proceed
while still preventing client releasing other caps.

Fixes: #9800
Signed-off-by: John Spray <john.spray@redhat.com>
Merge pull request #2809 from ceph/wip-9800

client: allow xattr caps in inject_release_failure

Reviewed-by: Greg Farnum <greg@inktank.com>