amd64: make uiomove_fromphys functional for pages not mapped by the DMAP

Place the code introduced in r268660 into a separate function that can be
called from uiomove_fromphys. Instead of pre-allocating two KVA pages use
vmem_alloc to allocate them on demand when needed. This prevents blocking if
a page fault is taken while physical addresses from outside the DMAP are
used, since the lock is now removed.

Also introduce a safety catch in PHYS_TO_DMAP and DMAP_TO_PHYS.

Sponsored by: Citrix Systems R&D
Reviewed by:		kib
Differential Revision:	https://reviews.freebsd.org/D947

amd64/amd64/pmap.c:
 - Factor out the code to deal with non DMAP addresses from pmap_copy_pages
   and place it in pmap_map_io_transient.
 - Change the code to use vmem_alloc instead of a set of pre-allocated
   pages.
 - Use pmap_qenter and don't pin the thread if there can be page faults.

amd64/amd64/uio_machdep.c:
 - Use pmap_map_io_transient in order to correctly deal with physical
   addresses not covered by the DMAP.

amd64/include/pmap.h:
 - Add the prototypes for the new functions.

amd64/include/vmparam.h:
 - Add safety catches to make sure PHYS_TO_DMAP and DMAP_TO_PHYS are only
   used with addresses covered by the DMAP.