cam/da: Only mark pack as valid if we know the size in daopen

Only mark the pack as 'valid' (eg clear the invalid bit) when we know
the size of the underlying drive and the periph hasn't been
invalidated. Previously, we'd unconditionally clear this bit, but
if the size isn't known, no I/O to the drive is possible.

Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D48688