cifs: Use generic_file_splice_read()

cifs: Use generic_file_splice_read()

Make cifs use generic_file_splice_read() rather than doing it for itself.

As a consequence, filemap_splice_read() no longer needs to be exported.

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Paulo Alcantara (SUSE) <pc@manguebit.com>
cc: Jens Axboe <axboe@kernel.dk>
cc: Steve French <smfrench@gmail.com>
cc: Al Viro <viro@zeniv.linux.org.uk>
cc: David Hildenbrand <david@redhat.com>
cc: John Hubbard <jhubbard@nvidia.com>
cc: linux-mm@kvack.org
cc: linux-block@vger.kernel.org
cc: linux-cifs@vger.kernel.org
cc: linux-fsdevel@vger.kernel.org
Link: https://lore.kernel.org/r/20230315163549.295454-2-dhowells@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>