MFC r253398:
Add a tunable to force disable MSI use for xhci(4).

Approved by:	re (delphij)
MFC r253280:
Only copy as much bytes as there in superblock, instead of the full
block copy, when copying the superblock into the snapshot.  UFS1 does
not align superblock on the block boundary, and bcopy runs off the end
of the buffer.

Approved by:	re (hrs)