Fix a number of races. First, retain PG_BUSY through a vm_page_remove(),

Fix a number of races.  First, retain PG_BUSY through a vm_page_remove(),
allowing it to be held atomically through a vm_page_rename() and
through most of a vm_page_free_toq() call.  Before vm_page_remove() unbusy'd
the page, leading to possible races and definite future issues.

Use a critical section instead of splvm() for critical VM page operations,
to protect against IPIs.

In-discussion-with: David Rhodus