um: Fix kmalloc argument order in um/vdso/vma.c

kmalloc size is 1st arg, not second.

Signed-off-by: Dave Jones <davej@redhat.com>
Signed-off-by: Richard Weinberger <richard@nod.at>

Cc: <stable@kernel.org> # 3.0.x
[richard@nod.at: on 3.0 the to be patched file is
arch/um/sys-x86_64/vdso/vma.c]