linuxkpi: Fix assertion in `lkpi_vmf_insert_pfn_prot_locked()`

[dumbbell]

Summary

Before this change, the assertion would not catch a page index matching the end address, even though the end address of a mapping is outside of that mapping.

While here, add another assertion to catch page indexes outside of the expected range earlier.

Also, pagefaulting an address outside of the mapping range should return VM_FAULT_SIGBUS instead of panicing.

Sponsored by: The FreeBSD Foundation