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
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D34944