f2fs: fix wrong available node count calculation

In mkfs, we have counted quota file's node number in cp.valid_node_count,
so we have to avoid wrong substraction of quota node number in
.available_nid/.avail_node_count calculation.

f2fs_write_check_point_pack()
{
..
	set_cp(valid_node_count, 1 + c.quota_inum + c.lpf_inum);

Fixes: 292c196a ("f2fs: reserve nid resource for quota sysfile")
Fixes: 7b63f72f ("f2fs: fix to do sanity check on valid node/block count")
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>