find_vmap_area() can return a NULL pointer and we're going to
dereference it without checking it first.  Use the existing
find_vm_area() function which does exactly what we want and checks for
the NULL pointer.

Link: http://lkml.kernel.org/r/20181228171009.22269-1-liviu@dudau.co.uk
Fixes: f3c01d2f ("mm: vmalloc: avoid racy handling of debugobjects in vunmap")
Signed-off-by: NLiviu Dudau <liviu@dudau.co.uk>
Reviewed-by: NAndrew Morton <akpm@linux-foundation.org>
Cc: Chintan Pandya <cpandya@codeaurora.org>
Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>