Declare u2mfn module version, skip build for qubes kernels

This will allow dkms to skip u2mfn module install if one is already
shipped with the kernel package - which is the case for kernels
delivered through dom0.
Make the version high enough to be considered newer than dkms package.

Sadly, it does not prevent module build. And the build fails becaue of
mismatching compiler version (kernel headers include gcc plugins).
Skip the build by setting BUILD_EXCLUSIVE_KERNEL in dkms.conf. Ideally,
we'd set some value indicating "don't build on kernel *qubes*", but
this variable does not support negation. So, set this variable to a
dummy value after manually checking $kernelver variable.

Fixes QubesOS/qubes-issues#4963

kernel-modules/u2mfn/dkms.conf.in

@@ -6,3 +6,7 @@ PACKAGE_NAME="u2mfn"
 BUILT_MODULE_NAME[0]="u2mfn"
 DEST_MODULE_LOCATION[0]="/extra/"
 AUTOINSTALL=yes
+# BUILD_EXCLUSIVE_KERNEL doesn't support negation :(
+if echo "$kernelver"|grep -q qubes; then
+    BUILD_EXCLUSIVE_KERNEL=not-on-qubes-kernel
+fi

kernel-modules/u2mfn/u2mfn.c

@@ -170,3 +170,4 @@ static void u2mfn_exit(void)
 module_init(u2mfn_init);
 module_exit(u2mfn_exit);
 MODULE_LICENSE("GPL");
+MODULE_VERSION("5.0.0");