FreeBSD: Simplify INGLOBALZONE

FreeBSD's previous ZFS implemented INGLOBALZONE(thread) as
(!jailed((thread)->td_ucred)) and passed curthread to INGLOBALZONE.

We pass curproc instead of curthread, so we can achieve the same effect
with (!jailed((proc)->p_ucred)). The implementation is trivial enough
to fit on a single line in a define. We don't really need a whole
separate function for something that's already macros all the way down.

Eliminate in_globalzone.

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <freqlabs@FreeBSD.org>
Closes #10851