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>
(cherry picked from commit 5691c68a0a44eb2cdf0afb3f39a540f5d42a5c0c)