aoe: module parameter for device timeout

The aoe_deadsecs module parameter sets the number of seconds that
elapse before a nonresponsive AoE device is marked as dead.

This is runtime settable in sysfs or settable with a module load or
kernel boot parameter.

Signed-off-by: "Ed L. Cashin" <ecashin@coraid.com>
Acked-by: Alan Cox <alan@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>