f2fs: zone: introduce first_zoned_segno in f2fs_sb_info

first_zoned_segno() returns a fixed value, let's cache it in
structure f2fs_sb_info to avoid redundant calculation.

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