buffer: use bdev_getblk() to avoid memory reclaim in readahead path

__getblk() adds __GFP_NOFAIL, which is unnecessary for readahead; we're
quite comfortable with the possibility that we may not get a bh back. 
Switch to bdev_getblk() which does not include __GFP_NOFAIL.

Link: https://lkml.kernel.org/r/20230914150011.843330-5-willy@infradead.orgSigned-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Hui Zhu <teawater@antgroup.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

fs/buffer.c

@@ -1465,7 +1465,9 @@ EXPORT_SYMBOL(__getblk_gfp);
  */
 void __breadahead(struct block_device *bdev, sector_t block, unsigned size)
 {
-	struct buffer_head *bh = __getblk(bdev, block, size);
+	struct buffer_head *bh = bdev_getblk(bdev, block, size,
+			GFP_NOWAIT | __GFP_MOVABLE);
+
 	if (likely(bh)) {
 		bh_readahead(bh, REQ_RAHEAD);
 		brelse(bh);