mm/vmalloc.c: don't dereference possible NULL pointer in __vunmap()

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.

Change-Id: I9b58c7423edb7927b64b722de886f8bc98c5f9ca
Git-commit: 6ade20327dbb808882888ed8ccded71e93067cf9
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
Link: http://lkml.kernel.org/r/20181228171009.22269-1-liviu@dudau.co.uk


Fixes: f3c01d2f3ade ("mm: vmalloc: avoid racy handling of debugobjects in vunmap")
Signed-off-by: Liviu Dudau <liviu@dudau.co.uk>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Chintan Pandya <cpandya@codeaurora.org>
Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Prakruthi Deepak Heragu <pheragu@codeaurora.org>