libprocstat: fix ZFS support

libprocstat: fix ZFS support

First of all, znode_phys_t hasn't been used for storing file attributes
for a long time now.  Modern ZFS versions use a System Attribute table
with a flexible layout.  But more importantly all the required
information is available in znode_t itself.

It's not easy to include zfs_znode.h in userland without breaking code
because the most interesting parts of the header are kernel-only. And
hardcoding field offsets is too fragile.  So, I created a new
compilation unit that includes zfs_znode.h using some mild kludges to
get it and its dependencies to compile in userland.  The compilation
unit exports interesting field offsets and does not have any other code.

PR:		194117
Reviewed by:	markj
MFC after:	2 weeks
Sponsored by:	Panzura
Differential Revision: https://reviews.freebsd.org/D24941