MFC r244575:
The manpage states that bus_dmamap_create(9) returns ENOMEM if it can't
allocate a map or mapping resources.  That seems to imply that any memory
allocations it does must use M_NOWAIT and check for NULL.

MFC 246158:
Use pmap_kextract() instead of inlining the page table walk.
Remove the comment referencing non-existing code.

Submitted by:	kib, cognet
Approved by:	marius
Obtained from:	Netflix