f2fs: clean up the start_bidx_of_node function

This patch also resolves the following warning reported by kbuild test robot.

fs/f2fs/gc.c: In function 'start_bidx_of_node':
fs/f2fs/gc.c:453:21: warning: 'bidx' may be used uninitialized in this function

Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>