mm: allow read-ahead with IOCB_NOWAIT set

The read-ahead shouldn't block, so allow it to be done even if
IOCB_NOWAIT is set in the kiocb.
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

mm/filemap.c

@@ -2028,8 +2028,6 @@ ssize_t generic_file_buffered_read(struct kiocb *iocb,
 
 		page = find_get_page(mapping, index);
 		if (!page) {
-			if (iocb->ki_flags & IOCB_NOWAIT)
-				goto would_block;
 			page_cache_sync_readahead(mapping,
 					ra, filp,
 					index, last_index - index);