f2fs: fix race condition in between free nid allocator/initializer

In below concurrent case, allocated nid can be loaded into free nid cache
and be allocated again.

Thread A				Thread B
- f2fs_create
 - f2fs_new_inode
  - alloc_nid
   - __insert_nid_to_list(ALLOC_NID_LIST)
					- f2fs_balance_fs_bg
					 - build_free_nids
					  - __build_free_nids
					   - scan_nat_page
					    - add_free_nid
					     - __lookup_nat_cache
 - f2fs_add_link
  - init_inode_metadata
   - new_inode_page
    - new_node_page
     - set_node_addr
 - alloc_nid_done
  - __remove_nid_from_list(ALLOC_NID_LIST)
					     - __insert_nid_to_list(FREE_NID_LIST)

This patch makes nat cache lookup and free nid list operation being atomical
to avoid this race condition.

Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
pgoff_t get_next_page_offset(struct dnode_of_data *dn, pgoff_t pgofs)
{
	const long direct_index = ADDRS_PER_INODE(dn->inode);
	const long direct_blks = ADDRS_PER_BLOCK;
	const long indirect_blks = ADDRS_PER_BLOCK * NIDS_PER_BLOCK;
	unsigned int skipped_unit = ADDRS_PER_BLOCK;
	int cur_level = dn->cur_level;
	int max_level = dn->max_level;
	pgoff_t base = 0;

	if (!dn->max_level)
		return pgofs + 1;

	while (max_level-- > cur_level)
		skipped_unit *= NIDS_PER_BLOCK;

	switch (dn->max_level) {
	case 3:
		base += 2 * indirect_blks;
	case 2:
		base += 2 * direct_blks;
	case 1:
		base += direct_index;
		break;
	default:
		f2fs_bug_on(F2FS_I_SB(dn->inode), 1);
	}

	return ((pgofs - base) / skipped_unit + 1) * skipped_unit + base;
}

pgoff_t get_next_page_offset(struct dnode_of_data *dn, pgoff_t pgofs)
{
	const long direct_index = ADDRS_PER_INODE(dn->inode);
	const long direct_blks = ADDRS_PER_BLOCK;
	const long indirect_blks = ADDRS_PER_BLOCK * NIDS_PER_BLOCK;
	unsigned int skipped_unit = ADDRS_PER_BLOCK;
	int cur_level = dn->cur_level;
	int max_level = dn->max_level;
	pgoff_t base = 0;

	if (!dn->max_level)
		return pgofs + 1;

	while (max_level-- > cur_level)
		skipped_unit *= NIDS_PER_BLOCK;

	switch (dn->max_level) {
	case 3:
		base += 2 * indirect_blks;
	case 2:
		base += 2 * direct_blks;
	case 1:
		base += direct_index;
		break;
	default:
		f2fs_bug_on(F2FS_I_SB(dn->inode), 1);
	}

	return ((pgofs - base) / skipped_unit + 1) * skipped_unit + base;
}
