ANDROID: Avoid get_order(0) in kmalloc_gfp_adjust tracepoint

The tracepoint `trace_android_vh_adjust_kvmalloc_flags` in
`kmalloc_gfp_adjust` is called with `get_order(size)`. Since `kvmalloc`
can be called with `size = 0`, this can result in `get_order(0)`, which
has undefined behavior.

Prevent this by adding a `size > 0` check before calling the tracepoint.

Bug: 450002836
Change-Id: Iaa76bbcf34a873971c422b97498181824bbd48fd
Signed-off-by: Jordan Chang <jordanchang@google.com>