cifs: check whether acl is valid early

Dan reported that acl is dereferenced before being checked and this is a
valid problem. Fix it be erroring out early instead of doing it later after
we've already relied on acl to be a valid pointer.

Fixes: dc1af4c4 ("cifs: implement set acl method")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Christian Brauner (Microsoft) <brauner@kernel.org>