KVM: selftests: Include sys/mman.h *and* linux/mman.h, via kvm_syscalls.h

KVM: selftests: Include sys/mman.h *and* linux/mman.h, via kvm_syscalls.h

Include both linux/mman.h (the kernel provided version) and sys/mman.h (the
libc provided version) throughout KVM selftests, by way of kvm_syscalls.h
(which should have been including sys/mman.h anyways).  Pulling in the
kernel's version fixes compilation errors with the guest_memfd test on
older versions of libc due to a recent commit adding MADV_COLLAPSE testing.

  In file included from include/kvm_util.h:8,
                   from guest_memfd_test.c:21:
  guest_memfd_test.c: In function ‘test_collapse’:
  guest_memfd_test.c:219:47: error: ‘MADV_COLLAPSE’ undeclared (first use in this function); did you mean ‘MADV_COLD’?
      219 |         TEST_ASSERT_EQ(madvise(mem, pmd_size, MADV_COLLAPSE), -1);
          |                                               ^~~~~~~~~~~~~
    include/test_util.h:62:16: note: in definition of macro ‘TEST_ASSERT_EQ’
       62 |         typeof(a) __a = (a);                                            \
          |                ^
    guest_memfd_test.c:219:47: note: each undeclared identifier is reported only once for each function it appears in
      219 |         TEST_ASSERT_EQ(madvise(mem, pmd_size, MADV_COLLAPSE), -1);
          |                                               ^~~~~~~~~~~~~
    include/test_util.h:62:16: note: in definition of macro ‘TEST_ASSERT_EQ’
       62 |         typeof(a) __a = (a);                                            \
          |                ^

Route the includes through kvm_syscalls.h to try and avoid a future game
of whack-a-mole, i.e. so that future expansion of test coverage doesn't run
into the same problem.

To discourage use of sys/mman.h, opportunistically include the kernel's
version of mman.h in test_util.h as it only needs MAP_SHARED, i.e. only
needs the full set of kernel defs, not the libc syscall wrappers.

Fixes: 9830209b4ae8 ("KVM: selftests: Test MADV_COLLAPSE on guest_memfd")
Reported-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Closes: https://lore.kernel.org/all/20260427204313.50741-1-rick.p.edgecombe@intel.com
Link: https://patch.msgid.link/20260428012503.1213654-1-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>