UBUNTU: [Packaging] disable zfs module checks when cross-compiling

When we cross-compile we have to turn off the inbuilt zfs modules.
When doing this we also need to allow the modules so produced to go
missing in this build while still checking the remainder of the modules.

BugLink: http://bugs.launchpad.net/bugs/1581127


Signed-off-by: Andy Whitcroft <apw@canonical.com>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>