Integer underflow in efipart_realstrategy when I/O starts after end of disk

Integer underflow in efipart_realstrategy when I/O starts after end of disk

This fixes an integer underflow in efipart_realstrategy, which causes
crashes when an I/O operation's start point is after the end of the disk.
This can happen when trying to detect filesystems on very small disks.
This can occur if a BIOS freebsd-boot partition exists on a system when the
EFI loader is being used.

PR:		219000
Submitted by:	Eric McCorkle <eric@metricspace.net>
Reviewed by:	cem (previous version), tsoome (previous version)
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D10559