f2fs: zone: introduce first_zoned_segno in f2fs_sb_info

[ Upstream commit 5bc5aae8 ]

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>
Stable-dep-of: dc6d9ef5 ("f2fs: zone: fix to calculate first_zoned_segno correctly")
Signed-off-by: Sasha Levin <sashal@kernel.org>