linuxkpi: Fix an assertion in linux_free_kmem()

In the LinuxKPI, PAGE_MASK is the logical negation of FreeBSD's
PAGE_MASK, so the original assertion was simply incorrect.

Reported by: trasz
Tested by: trasz
Fixes: 6223d0b67af9 ("linuxkpi: Handle direct-mapped addresses in linux_free_kmem()")