md(4): always trim the last partial sector

Do it also for the preloaded disk, in addition to the dynamically
configured device. This is needed to avoid geom checking alignment and
panicing on read of the last sector, e.g. for partition schemes and
label tasting.

PR: 281978
Reported by: bz
Reviewed by: bz, markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D47102