Remove rdev packing

Remove rdev packing

Remove custom code to pack/unpack dev_t's.  Under Linux all dev_t's
are an unsigned 32-bit value even on 64-bit platforms.  The lower
20 bits are used for the minor number and the upper 12 for the major
number.

This means if your importing a pool from Solaris you may get strange
major/minor numbers.  But it doesn't really matter because even if
we add compatibility code to translate the encoded Solaris major/minor
they won't do you any good under Linux.  You will still need to
recreate the dev_t with a major/minor which maps to reserved major
numbers used under Linux.

Dropping this code also resolves 32-bit builds by removing the
offending 32-bit compatibility code.