ext4: don't call update_backups() multiple times for the same bg

When performing an online resize, we add a bunch of groups at one time
in ext4_flex_group_add, so in most cases a lot of group descriptors
will be in the same group block. But in the end of this function,
update_backups will be called for every group descriptor and the same
block will be copied and journalled again and again. It is really a
waste.

Fix things so we only update a particular bg descriptor block once and
skip subsequent updates of the same block.

Signed-off-by: Tao Ma <[e-mail]>
Signed-off-by: "Theodore Ts'o" <[e-mail]>

fs/ext4/resize.c

···
 				   EXT4_DESC_PER_BLOCK(sb));
 		int meta_bg = EXT4_HAS_INCOMPAT_FEATURE(sb,
 				EXT4_FEATURE_INCOMPAT_META_BG);
+		sector_t old_gdb = 0;
 
 		update_backups(sb, sbi->s_sbh->b_blocknr, (char *)es,
 			       sizeof(struct ext4_super_block), 0);
···
 			struct buffer_head *gdb_bh;
 
 			gdb_bh = sbi->s_group_desc[gdb_num];
+			if (old_gdb == gdb_bh->b_blocknr)
+				continue;
 			update_backups(sb, gdb_bh->b_blocknr, gdb_bh->b_data,
 				       gdb_bh->b_size, meta_bg);
+			old_gdb = gdb_bh->b_blocknr;
 		}
 	}
 exit: