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>
Hmm, there's not really an existing constant that shows that CEPH_CAP_SXATTR part of a capability is 2 bits wide, is there?  One could get the three by doing CEPH_CAP_GSHARED | CEPH_CAP_GEXCL I suppose.