proc: fixup proc-pid-vm test

Silly sizeof(pointer) vs sizeof(uint8_t[]) bug.

Link: http://lkml.kernel.org/r/20190414123009.GA12971@avx2
Fixes: e483b020

 ("proc: test /proc/*/maps, smaps, smaps_rollup, statm")
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>