f2fs: fix unlocked nat set cache operation

nm_i->nat_tree_lock is used to sync both the operations of nat entry
cache tree and nat set cache tree, however, it isn't held when flush
nat entries during checkpoint which lead to potential race, this patch
fix it by holding the lock when gang lookup nat set cache and delete
item from nat set cache.

Signed-off-by: Wanpeng Li <wanpeng.li@linux.intel.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>