f2fs: fix max orphan inodes calculation

cp_payload is introduced for sit bitmap to support large volume, and it is
just after the block of f2fs_checkpoint + nat bitmap, so the first segment
should include F2FS_CP_PACKS + NR_CURSEG_TYPE + cp_payload + orphan blocks.
However, current max orphan inodes calculation don't consider cp_payload,
this patch fix it by reducing the number of cp_payload from total blocks of
the first segment when calculate max orphan inodes.

Signed-off-by: Wanpeng Li <wanpeng.li@linux.intel.com>
Reviewed-by: Chao Yu <chao2.yu@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>