x86/paravirt: Detect over-sized patching bugs in paravirt_patch_call()

paravirt_patch_call() currently handles patching failures inconsistently:
we generate a warning in the retpoline case, but don't in other cases where
we might end up with a non-working kernel as well.

So just convert it all to a BUG_ON(), these patching calls are *not* supposed
to fail, and if they do we want to know it immediately.

This also makes the kernel smaller and removes an #ifdef ugly.

I tried it with a richly paravirt-enabled kernel and no patching bugs
were detected.

Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20190425095039.GC115378@gmail.com


Signed-off-by: Ingo Molnar <mingo@kernel.org>