Pointy-hat to: me, for not realizing snprintf() is available in kernel.
Thanks to: jh, for bringing me the good news of snprintf(), Pawel Worach, for
noting that the panic can be provoked in i386 and not in amd64
implementation, error on the side of conservatism and only create labels
for GEOMs of classes DISK and MULTIPATH.
Discussed with: trasz
Approved by: silence from freebsd-geom@