tools/testing/selftests/proc/proc-pid-vm.c: test with vsyscall in mind

tools/testing/selftests/proc/proc-pid-vm.c: test with vsyscall in mind

: selftests: proc: proc-pid-vm
: ========================================
: proc-pid-vm: proc-pid-vm.c:277: main: Assertion `rv == strlen(buf0)' failed.
: Aborted

Because the vsyscall mapping is enabled.  Read from vsyscall page to tell
if vsyscall is being used.

Link: http://lkml.kernel.org/r/20190307183204.GA11405@avx2
Link: http://lkml.kernel.org/r/20190219094722.GB28258@shao2-debian
Fixes: 34aab6bec23e7e9 ("proc: test /proc/*/maps, smaps, smaps_rollup, statm")
Signed-off-by: Alexey Dobriyan <[email protected]>
Reported-by: kernel test robot <[email protected]>
Cc: Shuah Khan <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>