mm: kmemleak: avoid false negatives on vmalloc'ed objects

mm: kmemleak: avoid false negatives on vmalloc'ed objects

Commit 248ac0e1 ("mm/vmalloc: remove guard page from between vmap blocks")
had the side effect of making vmap_area.va_end member point to the next
vmap_area.va_start.  This was creating an artificial reference to
vmalloc'ed objects and kmemleak was rarely reporting vmalloc() leaks.

This patch marks the vmap_area containing pointers explicitly and reduces
the min ref_count to 2 as vm_struct still contains a reference to the
vmalloc'ed object.  The kmemleak add_scan_area() function has been
improved to allow a SIZE_MAX argument covering the rest of the object (for
simpler calling sites).

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>