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: Yan, Zheng <zyan@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>