f2fs: clean up some macros in terms of GET_SEGNO

This patch cleans several macros by introducing:
- BLKS_PER_SEC
- GET_SEC_FROM_SEG
- GET_SEG_FROM_SEC
- GET_ZONE_FROM_SEC
- GET_ZONE_FROM_SEG

Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>