mm, selftests: test return value of munmap for MAP_HUGETLB memory

When MAP_HUGETLB memory is unmapped, the length must be hugepage aligned,
otherwise it fails with -EINVAL.

All tests currently behave correctly, but it's better to explcitly test
the return value for completeness and document the requirement, especially
if users copy map_hugetlb.c as a sample implementation.

Signed-off-by: David Rientjes <rientjes@google.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Davide Libenzi <davidel@xmailserver.org>
Cc: Luiz Capitulino <lcapitulino@redhat.com>
Cc: Shuah Khan <shuahkh@osg.samsung.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Joern Engel <joern@logfs.org>
Cc: Jianguo Wu <wujianguo@huawei.com>
Cc: Eric B Munson <emunson@akamai.com>
Acked-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

tools/testing/selftests/powerpc/mm/hugetlb_vs_thp_test.c

@@ -21,9 +21,13 @@ static int test_body(void)
 		 * Typically the mmap will fail because no huge pages are
 		 * allocated on the system. But if there are huge pages
 		 * allocated the mmap will succeed. That's fine too, we just
-		 * munmap here before continuing.
+		 * munmap here before continuing.  munmap() length of
+		 * MAP_HUGETLB memory must be hugepage aligned.
 		 */
-		munmap(addr, SIZE);
+		if (munmap(addr, SIZE)) {
+			perror("munmap");
+			return 1;
+		}
 	}
 
 	p = mmap(addr, SIZE, PROT_READ | PROT_WRITE,