fusefs: fix a buffer overflow in the tests

The actual overflow occured in the ReadAhead.readahead test.
Surprisingly it has never segfaulted or resulted in any bad behavior.

Sponsored by: Axcient
Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D38718

(cherry picked from commit 0c9df4afc239ee52961443e95bca8be81f0dea9e)