These functions are similar and I would like to have their functionality
live in generic VM functions so that the details of vm_page
synchronization can be encapsulated in vm_page.c.

The change adds vm_page_release() and vm_page_release_locked(). They
both unwire the page and optionally attempt to free the page.
vm_page_release_locked() requires the object lock. The main difference
is that vfs_vmio_unwire() may attempt to free a mapped page, while
sendfile_free_page() does not. This change thus modifies the handling
of direct I/O and B_NOREUSE to avoid freeing a mapped page.