Remove a couple of VLAs

Remove a couple of VLAs

I found a couple of spots where VLAs are in use but where they can
easily be removed.

In one spot, adding 'const' is enough -- and is already done in
similar code elsewhere in the file.

In another spot, one of two arrays will be used, so making the buffer
large enough for both works.

Approved-By: John Baldwin <jhb@FreeBSD.org>