geli: Add a chicken switch for unmapped I/O

We have a report of a panic in GELI that appears to go away when
unmapped I/O is disabled. Add a tunable to make such investigations
easier in the future. No functional change intended.

PR: 262894
Reviewed by: asomers
Sponsored by: The FreeBSD Foundation

(cherry picked from commit 081b4452a758dd81dcdc68ffb6f7bad901d53e3d)