f2fs: fix wrong block address calculation for a split extent

This patch fixes wrong calculation on block address field when an extent is
split.

Reviewed-by: Chao Yu <chao2.yu@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
Signed-off-by: Samuel Pascua <pascua.samuel.14@gmail.com>