[PATCH] __bread oops fix

If a filesystem passes an idiotic blocksize into bread(), __getblk_slow() will
warn and will return NULL.  We have a report (from Hubert Tonneau
<hubert.tonneau@fullpliant.org>) of isofs_fill_super() doing this (passing in
a silly block size) against an unplugged CDROM drive.

But a couple of __getblk_slow() callers forgot to check for the NULL bh, hence
oops.
Signed-off-by: NAndrew Morton <akpm@osdl.org>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>

fs/buffer.c

@@ -1478,8 +1478,10 @@ EXPORT_SYMBOL(__getblk);
 void __breadahead(struct block_device *bdev, sector_t block, int size)
 {
 	struct buffer_head *bh = __getblk(bdev, block, size);
-	ll_rw_block(READA, 1, &bh);
-	brelse(bh);
+	if (likely(bh)) {
+		ll_rw_block(READA, 1, &bh);
+		brelse(bh);
+	}
 }
 EXPORT_SYMBOL(__breadahead);
 
@@ -1497,7 +1499,7 @@ __bread(struct block_device *bdev, sector_t block, int size)
 {
 	struct buffer_head *bh = __getblk(bdev, block, size);
 
-	if (!buffer_uptodate(bh))
+	if (likely(bh) && !buffer_uptodate(bh))
 		bh = __bread_slow(bh);
 	return bh;
 }