f2fs: cache NULL when both default_acl and acl are NULL

default_acl and acl of newly created inode will be initiated
as ACL_NOT_CACHED in vfs function inode_init_always() and later
will be updated by calling xxx_init_acl() in specific filesystems.
Howerver, when default_acl and acl are NULL then they keep the value
of ACL_NOT_CACHED, this patch tries to cache NULL for acl/default_acl
in this case.

Signed-off-by: Chengguang Xu <cgxu519@gmx.com>
Acked-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>