dtrace: Disable getf() as it is broken on FreeBSD

getf() on FreeBSD calls _sx_slock(), _sx_sunlock() and fget_locked().
Furthermore, it does not set the per-core fault flag, meaning it
usually ends up in a double fault panic once getf() does get called,
especially from fbt.

Reviewing the DTrace Toolkit + a number of other scripts scattered
around FreeBSD, I have not been able to find one use of getf(). Given
how broken the implementation currently is, we disable it until it
can be implemented properly.

Also comment out a test in aggs/tst.subr.d for getf().

Reviewed by: markj
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D33378