build: re-enable BTF

but allow discarding BTF information when loading modules, so that upgrades
which are otherwise ABI compatible still work. this allows using BTF
information when matching and available, while degrading gracefully if the
currently running kernel is not identical to the one that module was built for.

in case of a mismatch, the kernel will log a warning when loading the module,
for example:

Jan 30 13:57:58 test kernel: BPF:          type_id=184 bits_offset=4096
Jan 30 13:57:58 test kernel: BPF:
Jan 30 13:57:58 test kernel: BPF: Invalid name
Jan 30 13:57:58 test kernel: BPF:
Jan 30 13:57:58 test kernel: failed to validate module [bonding] BTF: -22

Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>

debian/rules

@@ -78,6 +78,7 @@ PVE_CONFIG_OPTS= \
 -d CONFIG_DEFAULT_SECURITY_DAC \
 -e CONFIG_DEFAULT_SECURITY_APPARMOR \
 --set-str CONFIG_DEFAULT_SECURITY apparmor \
+-e CONFIG_MODULE_ALLOW_BTF_MISMATCH \
 -d CONFIG_UNWINDER_ORC \
 -d CONFIG_UNWINDER_GUESS \
 -e CONFIG_UNWINDER_FRAME_POINTER \