ceph: rename unsafe_request_wait()

Rename it to flush_mdlog_and_wait_inode_unsafe_requests() to make
it more descriptive.

Signed-off-by: Xiubo Li <xiubli@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>