9p: do no return 0 from ->check_acl without actually checking

9p: do no return 0 from ->check_acl without actually checking

If we do not want to use ACLs we at least need to perform normal Unix
permission checks.  From the comment I'm not quite sure that's what
is intended, but if 0p wants to do permission checks entirely on the
server it needs to do so in ->permission, not in ->check_acl.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>