Several small fixes for the amd64 minidump code.

Several small fixes for the amd64 minidump code.

In report_progress(), use nitems(progress_track) instead of manually
hard-coding array size.  Wrap long line.

In blk_write(), code verifies that ptr and pa cannot be non-zero
simultaneously.  The later check for the page-alignment of the ptr
argument never triggers due to pa != 0 always implying ptr == NULL.  I
believe that the intent was to ensure that physicall address passed is
page-aligned, since the address is (temporary) mapped for the duration
of the page write.

Clear the progress_track.visited fields when starting minidump.  If
minidump is restarted or taken second time during the system lifetime,
progress is not printed otherwise, making operator suspectible to the
dump status.

Tested by:	pho
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week