f2fs: alloc new nids for xattr block in recovery

recovery file A:			recovery file B:
	-get_dnode_of_data
		-alloc_nid
					-recover_xattr_data
						-set_node_addr(sbi, &ni, NEW_ADDR, false);
							--->bug_on for nid has been used by file A

In recovery process, new allocated node blocks may "reuse" xattr block
nids, this patch alloc new nids for xattr blocks in recovery process to
avoid this problem.

Signed-off-by: Yunlei He <heyunlei@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>