ceph: voluntarily drop Xx caps for requests those touch parent mtime

For write requests the parent's mtime will be updated correspondingly.
And if the 'Xx' caps is issued and when releasing other caps together
with the write requests the MDS Locker will try to eval the xattr lock,
which need to change the locker state excl --> sync and need to do Xx
caps revocation.

Just voluntarily dropping CEPH_CAP_XATTR_EXCL caps to avoid a cap
revoke message, which could cause the mtime will be overwrote by stale
one.

[ idryomov: break unnecessarily long lines ]

Link: https://tracker.ceph.com/issues/61584


Signed-off-by: Xiubo Li <xiubli@redhat.com>
Reviewed-by: Milind Changire <mchangir@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>