mkfs.f2fs: remove build warning

Should be unsigned int for le32_to_cpu.

Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
diff --git a/mkfs/f2fs_format.c b/mkfs/f2fs_format.c
index a62a8fe..e300731 100644
--- a/mkfs/f2fs_format.c
+++ b/mkfs/f2fs_format.c
@@ -432,7 +432,7 @@
 	u_int32_t blk_size_bytes;
 	u_int64_t cp_seg_blk_offset = 0;
 	u_int32_t crc = 0;
-	int i;
+	unsigned int i;
 	char *cp_payload = NULL;
 
 	ckp = calloc(F2FS_BLKSIZE, 1);