CID 1009492: Logically dead code in sys/cam/scsi/scsi_xpt.c

CID 1009492: Logically dead code in sys/cam/scsi/scsi_xpt.c

In `probedone()`, for the `PROBE_REPORT_LUNS` case, all paths that
fall to the bottom of the case set `lp` to `NULL`, so the test for a
non-NULL value of `lp` and call to `free()` if true is dead code as
the test can never be true. Fix by eliminating the whole if
statement. To guard against a possible future change that accidentally
violates this assumption, use a `KASSERT()` to catch if `lp` is
non-NULL.

Reviewed by:	cem
MFC after:	1 week
Sponsored by:	Dell EMC Isilon
Differential Revision:	https://reviews.freebsd.org/D19109