tools/testing/selftests/proc/proc-pid-vm.c: hide "segfault at ffffffffff600000" dmesg...

tools/testing/selftests/proc/proc-pid-vm.c: hide "segfault at ffffffffff600000" dmesg spam

Test tries to access vsyscall page and if it doesn't exist gets SIGSEGV
which can spam into dmesg.  However the segfault happens by design.
Handle it and carry information via exit code to parent.

Link: http://lkml.kernel.org/r/20190524181256.GA2260@avx2
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>