Revert "livepatch: Remove reliable stacktrace check in klp_try_switch_task()"

This reverts commit 1d98a69e. Commit
31adf230 ("livepatch: Convert error about unsupported reliable
stacktrace into a warning") weakened the enforcement for architectures
to have reliable stack traces support. The system only warns now about
it.

It only makes sense to reintroduce the compile time checking in
klp_try_switch_task() again and bail out early.

Signed-off-by: Miroslav Benes <mbenes@suse.cz>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Reviewed-by: Kamalesh Babulal <kamalesh@linux.vnet.ibm.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>