selftests/vm: use kselftest skip code for skipped tests

There are several test cases in the vm directory are still using exit 0
when they need to be skipped.  Use the kselftest framework to skip code
instead so it can help us to distinguish the return status.

Criterion to filter out what should be fixed in vm directory:
  grep -r "exit 0" -B1 | grep -i skip

This change might cause some false-positives if people are running these
test scripts directly and only checking their return codes, which will
change from 0 to 4.  However I think the impact should be small as most of
our scripts here are already using this skip code.  And there will be no
such issue if running them with the kselftest framework.

Link: https://lkml.kernel.org/r/20210823073433.37653-1-po-hsu.lin@canonical.comSigned-off-by: Po-Hsu Lin <po-hsu.lin@canonical.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

tools/testing/selftests/vm/charge_reserved_hugetlb.sh

 #!/bin/sh
 # SPDX-License-Identifier: GPL-2.0
 
+# Kselftest framework requirement - SKIP code is 4.
+ksft_skip=4
+
 set -e
 
 if [[ $(id -u) -ne 0 ]]; then
   echo "This test must be run as root. Skipping..."
-  exit 0
+  exit $ksft_skip
 fi
 
 fault_limit_file=limit_in_bytes

tools/testing/selftests/vm/hugetlb_reparenting_test.sh

 #!/bin/bash
 # SPDX-License-Identifier: GPL-2.0
 
+# Kselftest framework requirement - SKIP code is 4.
+ksft_skip=4
+
 set -e
 
 if [[ $(id -u) -ne 0 ]]; then
   echo "This test must be run as root. Skipping..."
-  exit 0
+  exit $ksft_skip
 fi
 
 usage_file=usage_in_bytes