md(4): always trim the last partial sector

[kib]

Summary

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 label tasting.

PR:     281978

[bz]

I think the commit message might want a description that this does it for mdimage (loaded from loader) as well now avoiding a panic on boot.
For everything runtime this is NOP if I read it correctly?

Thanks a lot!

[markj]

I am surprised by this behaviour, it seems we should at least print a warning if the file is truncated.

[kib]

Add printf.

[markj]

sys/dev/md/md.c

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