KVM: selftests: Fix a semaphore imbalance in the dirty ring logging test

KVM: selftests: Fix a semaphore imbalance in the dirty ring logging test

When finishing the final iteration of dirty_log_test testcase, set
host_quit _before_ the final "continue" so that the vCPU worker doesn't
run an extra iteration, and delete the hack-a-fix of an extra "continue"
from the dirty ring testcase.  This fixes a bug where the extra post to
sem_vcpu_cont may not be consumed, which results in failures in subsequent
runs of the testcases.  The bug likely was missed during development as
x86 supports only a single "guest mode", i.e. there aren't any subsequent
testcases after the dirty ring test, because for_each_guest_mode() only
runs a single iteration.

For the regular dirty log testcases, letting the vCPU run one extra
iteration is a non-issue as the vCPU worker waits on sem_vcpu_cont if and
only if the worker is explicitly told to stop (vcpu_sync_stop_requested).
But for the dirty ring test, which needs to periodically stop the vCPU to
reap the dirty ring, letting the vCPU resume the guest _after_ the last
iteration means the vCPU will get stuck without an extra "continue".

However, blindly firing off an post to sem_vcpu_cont isn't guaranteed to
be consumed, e.g. if the vCPU worker sees host_quit==true before resuming
the guest.  This results in a dangling sem_vcpu_cont, which leads to
subsequent iterations getting out of sync, as the vCPU worker will
continue on before the main task is ready for it to resume the guest,
leading to a variety of asserts, e.g.

  ==== Test Assertion Failure ====
  dirty_log_test.c:384: dirty_ring_vcpu_ring_full
  pid=14854 tid=14854 errno=22 - Invalid argument
     1  0x00000000004033eb: dirty_ring_collect_dirty_pages at dirty_log_test.c:384
     2  0x0000000000402d27: log_mode_collect_dirty_pages at dirty_log_test.c:505
     3   (inlined by) run_test at dirty_log_test.c:802
     4  0x0000000000403dc7: for_each_guest_mode at guest_modes.c:100
     5  0x0000000000401dff: main at dirty_log_test.c:941 (discriminator 3)
     6  0x0000ffff9be173c7: ?? ??:0
     7  0x0000ffff9be1749f: ?? ??:0
     8  0x000000000040206f: _start at ??:?
  Didn't continue vcpu even without ring full

Alternatively, the test could simply reset the semaphores before each
testcase, but papering over hacks with more hacks usually ends in tears.

Reported-by: Shaoqin Huang <shahuang@redhat.com>
Fixes: 84292e565951 ("KVM: selftests: Add dirty ring buffer test")
Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Shaoqin Huang <shahuang@redhat.com>
Link: https://lore.kernel.org/r/20240202231831.354848-1-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>