ext4: mark block bitmap corrupted when found instead of BUGON

We already has similar code in ext4_mb_complex_scan_group(), but
ext4_mb_simple_scan_group() still affected.

Other reports: https://www.spinics.net/lists/linux-ext4/msg60231.html

Reviewed-by: Andreas Dilger <adilger@dilger.ca>
Signed-off-by: Dmitry Monakhov <dmonakhov@gmail.com>
Link: https://lore.kernel.org/r/20200310150156.641-1-dmonakhov@gmail.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/mballoc.c

@@ -1901,8 +1901,15 @@ void ext4_mb_simple_scan_group(struct ext4_allocation_context *ac,
 		BUG_ON(buddy == NULL);
 
 		k = mb_find_next_zero_bit(buddy, max, 0);
-		BUG_ON(k >= max);
-
+		if (k >= max) {
+			ext4_grp_locked_error(ac->ac_sb, e4b->bd_group, 0, 0,
+				"%d free clusters of order %d. But found 0",
+				grp->bb_counters[i], i);
+			ext4_mark_group_bitmap_corrupted(ac->ac_sb,
+					 e4b->bd_group,
+					EXT4_GROUP_INFO_BBITMAP_CORRUPT);
+			break;
+		}
 		ac->ac_found++;
 
 		ac->ac_b_ex.fe_len = 1 << i;