ceph: use default file splice read callback

Splice read/write implementation changed recently. When using
generic_file_splice_read(), iov_iter with type == ITER_PIPE is
passed to filesystem's read_iter callback. But ceph_sync_read()
can't serve ITER_PIPE iov_iter correctly (ITER_PIPE iov_iter
expects pages from page cache).

Fixing ceph_sync_read() requires a big patch. So use default
splice read callback for now.
Signed-off-by: NYan, Zheng <zyan@redhat.com>
Signed-off-by: NIlya Dryomov <idryomov@gmail.com>

fs/ceph/file.c

@@ -1770,7 +1770,6 @@ const struct file_operations ceph_file_fops = {
 	.fsync = ceph_fsync,
 	.lock = ceph_lock,
 	.flock = ceph_flock,
-	.splice_read = generic_file_splice_read,
 	.splice_write = iter_file_splice_write,
 	.unlocked_ioctl = ceph_ioctl,
 	.compat_ioctl	= ceph_ioctl,