Strip leading spaces off of a SCSI disk's serial number

sys/cam/scsi/scsi_xpt.c
Strip leading spaces off of a SCSI disk's reported serial number
when populating the CAM serial number. This affects the output of
"diskinfo -v" and the names of /dev/diskid/DISK-* device nodes,
among other things.

SPC5r05 says that the Product Serial Number field from the Unit
Serial Number VPD page is right-aligned. So any leading spaces are
not part of the actual serial number. Most devices don't left-pad
their serial numbers, but some do. In particular, the SN VPD page
that an LSI HBA emulates for a SATA drive contains enough
left-padding to fill a 20-byte field.

UPDATING
Add a note to UPDATING, because some users may have to update
/etc/fstab or geom labels.

Reviewed by: ken, mav
MFC after: Never
Sponsored by: Spectra Logic Corp
Differential Revision: https://reviews.freebsd.org/D6516