Add Linux Compat Infrastructure

Add Linux Compat Infrastructure

Lay the initial ground work for a include/linux/ compatibility
directory.  This was less critical in the past because the bulk
of the ZFS code consumes the Solaris API via the SPL.  This API
was stable and the bulk Linux API differences were handled in
the SPL.

However, with the addition of a full Posix layer written directly
against the Linux APIs we are going to need more compatibility
code.  It makes sense that all this code should be cleanly located
in one place.  Subsequent patches should move the existing zvol
and vdev_disk compatibility code in to this directory.