ext4: convert BUG_ON's to WARN_ON's in mballoc.c

If the in-core buddy bitmap gets corrupted (or out of sync with the
block bitmap), issue a WARN_ON and try to recover.  In most cases this
involves skipping trying to allocate out of a particular block group.
We can end up declaring the file system corrupted, which is fair,
since the file system probably should be checked before we proceed any
further.

Link: https://lore.kernel.org/r/20200414035649.293164-1-tytso@mit.edu
Google-Bug-Id: 34811296
Google-Bug-Id: 34639169
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/mballoc.c

@@ -1943,7 +1943,8 @@ void ext4_mb_complex_scan_group(struct ext4_allocation_context *ac,
 	int free;
 
 	free = e4b->bd_info->bb_free;
-	BUG_ON(free <= 0);
+	if (WARN_ON(free <= 0))
+		return;
 
 	i = e4b->bd_info->bb_first_free;
 
@@ -1966,7 +1967,8 @@ void ext4_mb_complex_scan_group(struct ext4_allocation_context *ac,
 		}
 
 		mb_find_extent(e4b, i, ac->ac_g_ex.fe_len, &ex);
-		BUG_ON(ex.fe_len <= 0);
+		if (WARN_ON(ex.fe_len <= 0))
+			break;
 		if (free < ex.fe_len) {
 			ext4_grp_locked_error(sb, e4b->bd_group, 0, 0,
 					"%d free clusters as per "