proc: test how it holds up with mapping'less process

Create process without mappings and check

	/proc/*/maps
	/proc/*/numa_maps
	/proc/*/smaps
	/proc/*/smaps_rollup

They must be empty (excluding vsyscall page) or full of zeroes.

Retroactively this test should've caught embarassing /proc/*/smaps_rollup
oops:

[17752.703567] BUG: kernel NULL pointer dereference, address: 0000000000000000
[17752.703580] #PF: supervisor read access in kernel mode
[17752.703583] #PF: error_code(0x0000) - not-present page
[17752.703587] PGD 0 P4D 0
[17752.703593] Oops: 0000 [#1] PREEMPT SMP PTI
[17752.703598] CPU: 0 PID: 60649 Comm: cat Tainted: G        W         5.19.9-100.fc35.x86_64 #1
[17752.703603] Hardware name: To Be Filled By O.E.M. To Be Filled By O.E.M./X99 Extreme6/3.1, BIOS P3.30 08/05/2016
[17752.703607] RIP: 0010:show_smaps_rollup+0x159/0x2e0

Note 1:
	ProtectionKey field in /proc/*/smaps is optional,
	so check most of its contents, not everything.

Note 2:
	due to the nature of this test, child process hardly can signal
	its readiness (after unmapping everything!) to parent.
	I feel like "sleep(1)" is justified.
	If you know how to do it without sleep please tell me.

Note 3:
	/proc/*/statm is not tested but can be.

Link: https://lkml.kernel.org/r/Yz3liL6Dn+n2SD8Q@localhost.localdomain
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>