ceph: properly drop caps for setattr request

For CEPH_SETATTR_ATIME, MDS needs to xlock filelock, Fsxrw caps
are not allowed for xlocked filelock.

For CEPH_SETATTR_SIZE request that truncates file to smaller size,
MDS needs to xlock filelock, Fsxrw caps are not allowed for xlocked
filelock.

Signed-off-by: "Yan, Zheng" <zyan@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>