f2fs: fix passing relative address when discard zones

We should not pass relative address in a zone to
__f2fs_issue_discard_zone().

Signed-off-by: Daeho Jeong <daehojeong@google.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>