The FreeBSD linux/compiler.h in OpenZFS was copied from a very old
version of FreeBSD's linuxkpi's linux/compiler.h. There's no need for
this duplication. Use FreeBSD's linuxkpi version instead, and provide
zfs_fallthrough to augment it (it's all that's needed). Use #pragma once
to avoid naming issues for guard variables. Since this is a complete
rewrite, use my copyright here (the original code in FreeBSD still
credits everybody).
Remove extra copies of macros that were defined elsewhere, but are now
properly defined in LinuxKPI so are redundant.