staging: unisys: visorbus: tolerate larger-than-expected controlvm channel

Use the dynamic size of the controlvm channel (struct channel_header.size)
instead of the statically computed sizeof(struct controlvm_channel) when
determining the valid bounds for visorchannel_read() and
visorchannel_write().

This prevents an observed problem where kdump was failing because
controlvm_channel.local_crash_msg_offset was pointing beyond the statically
computed size of the channel, even though the channel was physically large
enough.  This was causing visorchannel_read() to unecessarily fail, because
we thought we were attempting to access memory outside of the channel.

Signed-off-by: Timothy Sell <timothy.sell@unisys.com>
Signed-off-by: David Kershner <david.kershner@unisys.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>