The Data Byte Count (DBC) field of a Physical Region Descriptor
Table is 22 bits, with the bit 31 being the interrupt-on-completion
bit.

OpenBSD and UEFI set this bit, resulting in large block i/o lengths
being sent to bhyve and coredumping the process. Fix by masking off
the relevant 22 bits when using the DBC field as a length.

Reviewed by:	Zhixiang Yu
Discussed with:	Tycho Nightingale (tycho.nightingale@pluribusnetworks.com)
MFC after:	10.0