Use ATTR_DBM even when hardware dirty bit management is not enabled.

The ARMv8 reference manual only states that the bit is reserved in
this case; following Linux's example, use it instead of a
software-defined bit for the purpose of indicating that a managed
mapping is writable.

Reviewed by: alc, andrew
MFC after: r350004
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D21121