f2fs-tools: fix to avoid uninitialized variable access

Fix below compile warning:

  CC       fibmap.o
In function ‘stat_bdev’,
    inlined from ‘main’ at fibmap.c:171:2:
fibmap.c:123:9: warning: ‘start_lba’ may be used uninitialized [-Wmaybe-uninitialized]
  123 |         printf("start_lba = %u\n", *start_lba);
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
fibmap.c: In function ‘main’:
fibmap.c:143:15: note: ‘start_lba’ was declared here
  143 |         __u32 start_lba;
      |               ^~~~~~~~~

Signed-off-by: Chao Yu <chao@kernel.org>