geli: use unmapped I/O

Use unmapped I/O for geli. Unlike most geom providers, geli needs to
manipulate data on every read or write. Previously it would always map bios.

On my 16-core, dual socket server using geli atop md(4) devices, with 512B
sectors, this change increases geli IOPs by about 3x.

Note that geli still can't use unmapped I/O when data integrity verification
is enabled (but it could, with a little more work). And it can't use
unmapped I/O in combination with ZFS, because ZFS uses mapped bios.

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