ext4: mballoc: avoid garbage value from err

clang's static analysis warning: fs/ext4/mballoc.c
line 4178, column 6, Branch condition evaluates to a garbage value.

err is uninitialized and will be judged when 'len <= 0' or
it first enters the loop while the condition "!ext4_sb_block_valid()"
is true. Although this can't make problems now, it's better to
correct it.
Signed-off-by: Su Hui <suhui@nfschina.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Link: https://lore.kernel.org/r/20230725043310.1227621-1-suhui@nfschina.comSigned-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/mballoc.c

@@ -4087,7 +4087,7 @@ void ext4_mb_mark_bb(struct super_block *sb, ext4_fsblk_t block,
 	struct ext4_sb_info *sbi = EXT4_SB(sb);
 	ext4_group_t group;
 	ext4_grpblk_t blkoff;
-	int i, err;
+	int i, err = 0;
 	int already;
 	unsigned int clen, clen_changed, thisgrp_len;