Fix some filemon path logging issues.

• Properly handle snprintf return value for truncation and avoid overflowing the later write with the bogus length.
• Increase the msgbufr size to handle a rename of 2 full files.

The larger allocation causes a slight performance hit which will be mitigated
in the future. A rewrite with sbufs will likely be done as well.

Reported by: Ilja Van Sprundel <ivansprundel@ioactive.com>
MFC after: 2 weeks
Approved by: so (gtetlow)
Reviewed by: kib
Sponsored by: Dell EMC
Differential Revision: https://reviews.freebsd.org/D16098