fs/buffer: avoid redundant lookup in getblk slowpath

__getblk_slow() already implies failing a first lookup
as the fastpath, so try to create the buffers immediately
and avoid the redundant lookup. This saves 5-10% of the
total cost/latency of the slowpath.

Signed-off-by: Davidlohr Bueso <dave@stgolabs.net>
Link: https://lore.kernel.org/20250515173925.147823-3-dave@stgolabs.net
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: Davidlohr Bueso

fs/buffer.c

@@ -1139,15 +1139,15 @@ __getblk_slow(struct block_device *bdev, sector_t block,
 	for (;;) {
 		struct buffer_head *bh;
 
+		if (!grow_buffers(bdev, block, size, gfp))
+			return NULL;
+
 		if (blocking)
 			bh = __find_get_block_nonatomic(bdev, block, size);
 		else
 			bh = __find_get_block(bdev, block, size);
 		if (bh)
 			return bh;
-
-		if (!grow_buffers(bdev, block, size, gfp))
-			return NULL;
 	}
 }