loader: add support for gzip compression

As we do have zlib code in loader, we should also support gzip
compression in zfs.

PR: 153173
Submitted by: Mikhail Zakharov <zmey20000@yahoo.com>
Reviewed by: imp, markj, delphij
Differential Revision: https://reviews.freebsd.org/D35320
MFC after: 1 month