Change get_pv_entry() such that the call to vm_page_alloc() specifies

Change get_pv_entry() such that the call to vm_page_alloc() specifies
VM_ALLOC_NORMAL instead of VM_ALLOC_SYSTEM when try is TRUE.  In other
words, when get_pv_entry() is permitted to fail, it no longer tries as
hard to allocate a page.

Change pmap_enter_quick_locked() to fail rather than wait if it is
unable to allocate a page table page.  This prevents a race between
pmap_enter_object() and the page daemon.  Specifically, an inactive
page that is a successor to the page that was given to
pmap_enter_quick_locked() might become a cache page while
pmap_enter_quick_locked() waits and later pmap_enter_object() maps
the cache page violating the invariant that cache pages are never
mapped.  Similarly, change
pmap_enter_quick_locked() to call pmap_try_insert_pv_entry() rather
than pmap_insert_entry().  Generally speaking,
pmap_enter_quick_locked() is used to create speculative mappings.  So,
it should not try hard to allocate memory if free memory is scarce.

Add an assertion that the object containing m_start is locked in
pmap_enter_object().  Remove a similar assertion from
pmap_enter_quick_locked() because that function no longer accesses the
containing object.

Remove a stale comment.

Reviewed by: ups@