Reduce allocations and possible failures in uvm_pagermapin/out().

Reduce allocations and possible failures in uvm_pagermapin/out().

. If a direct map exists use it to map single-page allocations
. Use pmap_kenter_pa() instead of pmap_enter() in all other cases.

This speeds up file-based mmap up to 75% when I/O are performed and it
also reduces possible allocations failtures in the page daemon making
it more stable in OOM situations.

ok kettenis@, beck@