ext4: replace BUG_ON with WARN_ON in mb_find_extent()

The last BUG_ON in mb_find_extent() is apparently triggering in some
rare cases.  Most of the time it indicates a bug in the buddy bitmap
algorithms, but there are some weird cases where it can trigger when
buddy bitmap is still in memory, but the block bitmap has to be read
from disk, and there is disk or memory corruption such that the block
bitmap and the buddy bitmap are out of sync.

Google-Bug-Id: #33702157
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/mballoc.c

@@ -1556,7 +1556,17 @@ static int mb_find_extent(struct ext4_buddy *e4b, int block,
 		ex->fe_len += 1 << order;
 	}
 
-	BUG_ON(ex->fe_start + ex->fe_len > (1 << (e4b->bd_blkbits + 3)));
+	if (ex->fe_start + ex->fe_len > (1 << (e4b->bd_blkbits + 3))) {
+		/* Should never happen! (but apparently sometimes does?!?) */
+		WARN_ON(1);
+		ext4_error(e4b->bd_sb, "corruption or bug in mb_find_extent "
+			   "block=%d, order=%d needed=%d ex=%u/%d/%d@%u",
+			   block, order, needed, ex->fe_group, ex->fe_start,
+			   ex->fe_len, ex->fe_logical);
+		ex->fe_len = 0;
+		ex->fe_start = 0;
+		ex->fe_group = 0;
+	}
 	return ex->fe_len;
 }