cifs: fix rmmod regression in cifs.ko caused by force_sig changes

Fixes: 72abe3bc ("signal/cifs: Fix cifs_put_tcp_session to call send_sig instead of force_sig")

The global change from force_sig caused module unloading of cifs.ko
to fail (since the cifsd process could not be killed, "rmmod cifs"
now would always fail)

Signed-off-by: Steve French <stfrench@microsoft.com>
Reviewed-by: Ronnie Sahlberg <lsahlber@redhat.com>
CC: Eric W. Biederman <ebiederm@xmission.com>