Use /dev/fd0a rather than /dev/rfd0a to read instfs.tgz image from floppy.
It looks newer (appearred after 1.6) gzip tries to read less than DEV_BSIZE
(to check header?) so we can't use raw device directly.
(note sparc bootfs ramdisk doesn't have dd(1))

Workaround for PR port-sparc/42193, and would also fix PR install/28734.