cifs: add local server pointer to cifs_setup_session

cifs_setup_session references pSesInfo->server several times. That
pointer shouldn't change during the life of the function so grab it
once and store it in a local var. This makes the code look a little
cleaner too.

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