ext4: 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.  However,
when default_acl and acl are NULL then they keep the value of
ACL_NOT_CACHED.  This patch changes the code to cache NULL for acl /
default_acl in this case to save unnecessary ACL lookup attempt.

Signed-off-by: Chengguang Xu <cgxu519@gmx.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Jan Kara <jack@suse.cz>