Notify userspace listeners when geom disk attributes have changed

[asomers]

Summary

Notify userspace listeners when geom disk attributes have changed.

sys/cam/scsi/scsi_da.c:

• When modifying softc->disk, check to see whether any fields have actually been updated. If so, call disk_attr_changed() with an attribute of "GEOM::diskparams".
• The ATA equivalent would not benefit from this, because it does not asynchronously update disk parameters; they are set prior to the disk_create() call in cam/ata/ata_da.c:adaregister().

sys/geom/geom_disk.c:

• disk_attr_changed(): Generate a devctl event of type GEOM:<attr> for every call.

Test Plan

Set devd's loglevel to info, then insert a disk into a SAS expander. /var/log/devd.log should show something like this:

Processing event '!system=GEOM subsystem=disk type=GEOM::diskparams devname=da0

[asomers]

sys/cam/scsi/scsi_da.c

3845 ↗

(On Diff #15198)

Currently, nothing cares about diskparams events. I could revert this part of the change, if my reviewers don't think diskparams events are worthwhile.

sys/geom/geom_disk.c

850 ↗

(On Diff #15198)

This is the most important part of the change. Without this, zfsd won't see physpath events, so autoreplace operations won't be reliable.

[asomers]

Decide to revert the diskparams event. Nothing's using it ATM and we can always bring it back later if we want.

[asomers]

Hearing no feedback for a week, I'll go ahead and commit.