ceph_aio_write(): switch to generic_perform_write()

			goto retry_snap;

		}

	} else {

		struct iov_iter from;

		/*

		 * No need to acquire the i_truncate_mutex. Because

		 * the MDS revokes Fwb caps before sending truncate

		 * are pending vmtruncate. So write and vmtruncate

		 * can not run at the same time

		 */

		written = generic_file_buffered_write(iocb, iov, nr_segs,

						      pos, count, 0);

		iov_iter_init(&from, iov, nr_segs, count, 0);

		written = generic_perform_write(file, &from, pos);

		if (likely(written >= 0))

			iocb->ki_pos = pos + written;

		mutex_unlock(&inode->i_mutex);

	}