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>
