cifs: mangle existing header for SMB_COM_NT_CANCEL

The NT_CANCEL command looks just like the original command, except for a
few small differences. The send_nt_cancel function however currently takes
a tcon, which we don't have in SendReceive and SendReceive2.

Instead of "respinning" the entire header for an NT_CANCEL, just mangle
the existing header by replacing just the fields we need. This means we
don't need a tcon and allows us to call it from other places.

Reviewed-by: Pavel Shilovsky <piastryyy@gmail.com>
Reviewed-by: Suresh Jayaraman <sjayaraman@suse.de>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>