Fix arm64's pmap_enter_quick_locked()

[alc]

Summary

When arm64's pmap_enter_quick_locked() creates a user-space mapping, it doesn't set ATTR_AP(ATTR_AP_USER) in the PTE, so any subsequent attempt to read from the page triggers a page fault.

This problem isn't fatal because the page fault handler will determine that the mapping should be valid and perform a pmap_enter() that sets ATTR_AP(ATTR_AP_USER) in the new PTE.

That said, I do believe that instruction fetches are not affected by this bug. (See Table D5-33 in the architecture manual.) In other words, as long as the mapping doesn't have ATTR_XN set, user-space instruction fetches will succeed even without ATTR_AP(ATTR_AP_USER) in the PTE.

Test Plan

Clang reads source files using mmap(). Before applying this patch, vmstat -s reported "235989057 total VM faults taken" after a "buildworld". After applying the patch, vmstat -s reported "190243137 total VM faults taken".

I'm double-checking these numbers, because the difference is more than I would have guessed.

[markj]

Ugh. I fixed the same bug in RISC-V in r342733 and didn't think to look at arm64.