Abstract UIO allocation and deallocation.

Introduce the allocuio() and freeuio() functions to allocate and
deallocate struct uio. This hides the actual allocator interface, so it
is easier to modify the sub-allocation layout of struct uio and the
corresponding iovec array.

Obtained from: CheriBSD
Reviewed by: kib, markj
MFC after: 2 weeks
Sponsored by: CHaOS, EPSRC grant EP/V000292/1
Differential Revision: https://reviews.freebsd.org/D43711

(cherry picked from commit 61cc4830a7b16400efade3d884a59fda6d80d651)