ceph: remove superfluous inode_lock in ceph_fsync

Originally, filemap_write_and_wait took the i_mutex internally, but
commit 02c24a82 pushed the mutex acquisition into the individual
fsync routines, leaving it up to the subsystem maintainers to remove
it if it wasn't needed.

For ceph, I see no reason to take the inode_lock here. All of the
operations inside that lock are protected by their own locking.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: "Yan, Zheng" <zyan@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>