bio aio: Destroy ephemeral mapping before unwiring page.

Apparently some architectures, like ppc in its hashed page tables
variants, account mappings by pmap_qenter() in the response from
pmap_is_page_mapped().

While there, eliminate useless userp variable.

Noted and reviewed by: alc (previous version)
Sponsored by: The FreeBSD Foundation
Differential revision: https://reviews.freebsd.org/D27409