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.

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

(cherry picked from commit f88bd1174aab1aff7fea7241ab6e103e769d2d7a)
(cherry picked from commit 501e1152d2a047f8ff8fba6e6efbd2d5e9872738)