cifs: on multiuser mount, set ownership to current_fsuid/current_fsgid (try #5)

...when unix extensions aren't enabled. This makes everything on the
mount appear to be owned by the current user.

This version of the patch differs from previous versions however in that
the admin can still force the ownership of all files to appear as a
single user via the uid=/gid= options.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>