KVM: selftests: Fix build of rseq test

The KVM rseq test is failing to build in -next due to a commit merged
from the tip tree which adds a wrapper for sys_getcpu() to the rseq
kselftests, conflicting with the wrapper already included in the KVM
selftest:

rseq_test.c:48:13: error: conflicting types for 'sys_getcpu'
   48 | static void sys_getcpu(unsigned *cpu)
          |             ^~~~~~~~~~
In file included from rseq_test.c:23:
../rseq/rseq.c:82:12: note: previous definition of 'sys_getcpu' was here
   82 | static int sys_getcpu(unsigned *cpu, unsigned *node)
          |            ^~~~~~~~~~

Fix this by removing the local wrapper and moving the result check up to
the caller.

Fixes: 99babd04

 ("selftests/rseq: Implement rseq numa node id field selftest")
Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Link: https://lore.kernel.org/r/20230106-fix-kvm-rseq-build-v1-1-b704d9831d02@kernel.org