tests/fusefs: fix all tests that depend on kern.maxphys

The tests try to read kern.maxphys sysctl into int value, while
unsigned long is required. Not sure when this was broken, seems like
since cd8537910406e.

Reviewed by: asomers
Differential Revision: https://reviews.freebsd.org/D45053

(cherry picked from commit e9b411d273336647e61704213964b995952a44fd)