From: Hugh Dickins <hugh@veritas.com>

The prio_tree is of no use to nonlinear vmas: currently we're having to search
the tree in the most inefficient way to find all its nonlinears.  At the very
least we need an indication of the unlikely case when there are some
nonlinears; but really, we'd do best to take them out of the prio_tree
altogether, into a list of their own - i_mmap_nonlinear.