geom: use bool for one-bit wide bit-field

A one-bit wide bit-field can take only the values 0 and -1. Clang 16
introduced a warning that "implicit truncation from 'int' to a one-bit
wide bit-field changes value from 1 to -1". Fix by using c99 bool.

Reported by: Clang, via dim
Reviewed by: dim
Sponsored by: The FreeBSD Foundation