f2fs: clean up the unused variable additional_reserved_segments

additional_reserved_segments was introduced by
commit 300a8429 ("f2fs: fix to reserve space for IO align feature"),
and its initialization was deleted by
commit 87161a2b ("f2fs: deprecate io_bits").

Signed-off-by: LongPing Wei <weilongping@oppo.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>