lib/test_overflow.c: avoid tainting the kernel and fix wrap size

This adds __GFP_NOWARN to the kmalloc()-portions of the overflow test to
avoid tainting the kernel.  Additionally fixes up the math on wrap size
to be architecture and page size agnostic.

Link: http://lkml.kernel.org/r/201905282012.0A8767E24@keescook
Fixes: ca90800a

 ("test_overflow: Add memory allocation overflow tests")
Signed-off-by: Kees Cook <keescook@chromium.org>
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Suggested-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Cc: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>