mount.cifs: fix argument count check

mount.cifs: fix argument count check

The argv < 3 check could return true if you pass in some option flags.
If you don't provide any further arguments then you might just walk
off the end of the argv array. The values past the end aren't
guaranteed to be NULL in that case.

Fix the check to just look at whether there are 2 more arguments after
the getopt processing is done.

Signed-off-by: Jeff Layton <jlayton@samba.org>