Eliminate redundant calls to critical_enter() and critical_exit() from
pmap_update_entry().  It suffices that interrupts are blocked.

In short, pmap_enter_quick_locked("user space", ..., VM_PROT_READ) doesn't
work.  More precisely, it doesn't set ATTR_AP(ATTR_AP_USER) in the page
table entry, so any attempt to read from the mapped page by user space
generates a page fault.  This problem has gone unnoticed because the page
fault handler, vm_fault(), will ultimately call pmap_enter(), which
replaces the non-working page table entry with one that has
ATTR_AP(ATTR_AP_USER) set.  This change reduces the number of page faults
during a "buildworld" by about 19.4%.

Eliminate a redundant pmap_load() from pmap_remove_pages().

There is no reason why the pmap_invalidate_all() in pmap_remove_pages()
must be performed before the final PV list lock release.  Move it past
the lock release.

Eliminate a stale comment from pmap_page_test_mappings().  We implemented
a modified bit in r350004.