MFS r285951: Avoid creating invalid UEFI device path

MFS r285951: Avoid creating invalid UEFI device path

The UEFI loader on the 10.1 release install disk (disc1) modifies an
existing EFI_DEVICE_PATH_PROTOCOL instance in an apparent attempt to
truncate the device path.  In doing so it creates an invalid device
path.

Perform the equivalent action without modification of structures
allocated by firmware.

PR:		197641
Submitted by:	Chris Ruffin <chris.ruffin at intel.com>
Approved by:	re (gjb)

git-svn-id: https://svn.freebsd.org/base/releng/10.2@285956 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f