Change len checks for fstypelen and fspathlen to be against absolute len
not strlen as they are *not* strings.

Discovered by GSOC student, Mike Ma <mikemandarine@gmail.com> during his
fuse.glusterfs port to FreeBSD.

Final patch from mckusick@

Submitted by:	mckusick@
Approved by:	re (hrs)
MFC after:	2 weeks