ceph: refactor remove_session_caps_cb

Move remove_capsnaps to caps.c. Move the part of remove_session_caps_cb
under i_ceph_lock into a separate function that lives in caps.c. Have
remove_session_caps_cb call the new helper after taking the lock.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>