ceph: voluntarily drop Fx cap for readdir request

MDS need to rdlock directory inode's filelock when handling readdir
request. Voluntarily dropping CEPH_CAP_AUTH_EXCL avoids a cap revoke
message.

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