zonectl: Consistently report ZAC conv. zones WP LBA

On ZAC drives, conventional zones conventionally report a write pointer
LBA of 0xffffffffffff. This field is 48 bits wide, unlike ZBC's 64 bits.

Recognize both ZAC and ZBC all-ones behaviour in the WRITE POINTER LBA
field to indicate non-valid information.

Tested by: fuz
Discussed with: fuz, asomers, ken
Fixes: 4735ef6196bc ("zonectl: display conventional zones better")
MFC after: 2 weeks
Sponsored by: Google Summer Of Code 2026
Reviewed by: asomers
Pull Request: https://github.com/freebsd/freebsd-src/pull/2345