crypto(9): add CRYPTO_BUF_VMPAGE

crypto(9) functions can now be used on buffers composed of an array of
vm_page_t structures, such as those stored in an unmapped struct bio. It
requires the running to kernel to support the direct memory map, so not all
architectures can use it.

Reviewed by: markj, kib, jhb, mjg, mat, bcr (manpages)
MFC after: 1 week
Sponsored by: Axcient
Differential Revision: https://reviews.freebsd.org/D25671