ext4: fix online resizing for bigalloc file systems with a 1k block size

An online resize of a file system with the bigalloc feature enabled
and a 1k block size would be refused since ext4_resize_begin() did not
understand s_first_data_block is 0 for all bigalloc file systems, even
when the block size is 1k.
Signed-off-by: NTheodore Ts'o <tytso@mit.edu>
Cc: stable@vger.kernel.org

fs/ext4/resize.c

@@ -19,6 +19,7 @@
 
 int ext4_resize_begin(struct super_block *sb)
 {
+	struct ext4_sb_info *sbi = EXT4_SB(sb);
 	int ret = 0;
 
 	if (!capable(CAP_SYS_RESOURCE))
@@ -29,7 +30,7 @@ int ext4_resize_begin(struct super_block *sb)
          * because the user tools have no way of handling this.  Probably a
          * bad time to do it anyways.
          */
-	if (EXT4_SB(sb)->s_sbh->b_blocknr !=
+	if (EXT4_B2C(sbi, sbi->s_sbh->b_blocknr) !=
 	    le32_to_cpu(EXT4_SB(sb)->s_es->s_first_data_block)) {
 		ext4_warning(sb, "won't resize using backup superblock at %llu",
 			(unsigned long long)EXT4_SB(sb)->s_sbh->b_blocknr);