proc: fix PIE proc-empty-vm, proc-pid-vm tests

proc: fix PIE proc-empty-vm, proc-pid-vm tests

vsyscall detection code uses direct call to the beginning of
the vsyscall page:

	asm ("call %P0" :: "i" (0xffffffffff600000))

It generates "call rel32" instruction but it is not relocated if binary
is PIE, so binary segfaults into random userspace address and vsyscall
page status is detected incorrectly.

Do more direct:

	asm ("call *%rax")

which doesn't do need any relocaltions.

Mark g_vsyscall as volatile for a good measure, I didn't find instruction
setting it to 0. Now the code is obviously correct:

	xor	eax, eax
	mov	rdi, rbp
	mov	rsi, rbp
	mov	DWORD PTR [rip+0x2d15], eax      # g_vsyscall = 0
	mov	rax, 0xffffffffff600000
	call	rax
	mov	DWORD PTR [rip+0x2d02], 1        # g_vsyscall = 1
	mov	eax, DWORD PTR ds:0xffffffffff600000
	mov	DWORD PTR [rip+0x2cf1], 2        # g_vsyscall = 2
	mov	edi, [rip+0x2ceb]                # exit(g_vsyscall)
	call	exit

Note: fixed proc-empty-vm test oopses 5.19.0-28-generic kernel
	but this is separate story.

Link: https://lkml.kernel.org/r/Y7h2xvzKLg36DSq8@p183
Fixes: 5bc73bb3451b9 ("proc: test how it holds up with mapping'less process")
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Reported-by: Mirsad Goran Todorovac <mirsad.todorovac@alu.unizg.hr>
Tested-by: Mirsad Goran Todorovac <mirsad.todorovac@alu.unizg.hr>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>