selftests/mm: fix guard-pages build

JIRA: https://issues.redhat.com/browse/RHEL-78135
Upstream Status: RHEL only

The tools/testing/selftests/mm/guard-pages.c build was br0ken in v6.13:

"""
guard-pages.c: In function ‘guard_pages_process_madvise’:
guard-pages.c:422:17: warning: implicit declaration of function ‘process_madvise’; did you mean ‘SYS_process_madvise’? [-Wimplicit-function-declaration]
  422 |         count = process_madvise(pidfd, vec, 6, MADV_GUARD_INSTALL, 0);
      |                 ^~~~~~~~~~~~~~~
      |                 SYS_process_madvise
/usr/bin/ld: /tmp/ccLfVeGh.o: in function `guard_pages_process_madvise':
guard-pages.c:(.text+0x5c54): undefined reference to `process_madvise'
/usr/bin/ld: guard-pages.c:(.text+0x629c): undefined reference to `process_madvise'
collect2: error: ld returned 1 exit status
make: *** [../lib.mk:222: /home/luizcap/src/linux/tools/testing/selftests/mm/guard-pages] Error 1
"""

This is fixed in upstream commit 19b65ff ("selftests/mm: add fork
CoW guard page test"). Now, the problem is that that commit was merged
in v6.14-rc1 and it adds a new test too so I can't tell if the new test
is supported in v6.13. To be safe, do a partial backport of the build
fix only.

Signed-off-by: Luiz Capitulino <luizcap@redhat.com>

Author: luiz-cap

tools/testing/selftests/mm/guard-pages.c

@@ -55,6 +55,12 @@ static int pidfd_open(pid_t pid, unsigned int flags)
 	return syscall(SYS_pidfd_open, pid, flags);
 }
 
+static ssize_t sys_process_madvise(int pidfd, const struct iovec *iovec,
+				   size_t n, int advice, unsigned int flags)
+{
+	return syscall(__NR_process_madvise, pidfd, iovec, n, advice, flags);
+}
+
 /*
  * Enable our signal catcher and try to read/write the specified buffer. The
  * return value indicates whether the read/write succeeds without a fatal
@@ -419,7 +425,7 @@ TEST_F(guard_pages, process_madvise)
 	ASSERT_EQ(munmap(&ptr_region[99 * page_size], page_size), 0);
 
 	/* Now guard in one step. */
-	count = process_madvise(pidfd, vec, 6, MADV_GUARD_INSTALL, 0);
+	count = sys_process_madvise(pidfd, vec, 6, MADV_GUARD_INSTALL, 0);
 
 	/* OK we don't have permission to do this, skip. */
 	if (count == -1 && errno == EPERM)
@@ -440,7 +446,7 @@ TEST_F(guard_pages, process_madvise)
 	ASSERT_FALSE(try_read_write_buf(&ptr3[19 * page_size]));
 
 	/* Now do the same with unguard... */
-	count = process_madvise(pidfd, vec, 6, MADV_GUARD_REMOVE, 0);
+	count = sys_process_madvise(pidfd, vec, 6, MADV_GUARD_REMOVE, 0);
 
 	/* ...and everything should now succeed. */