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