[CIFS] properly account for new user= field in SPNEGO upcall string allocation

...it doesn't look like it's being accounted for at the moment. Also
try to reorganize the calculation to make it a little more evident
what each piece means.

This should probably go to the stable series as well...

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