Fix a kernel panic in DTrace's rw_iswriter subroutine.
On FreeBSD the sense of rw_write_held() and rw_iswriter() were reversed,
probably due to a cut and paste error. Using rw_iswriter() would cause
the kernel to panic.

Reviewed by: markj
MFC after: 2 weeks
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D8718