geom_part: extend kern.geom.part.check_integrity to work on GPT

There are multiple USB/SATA bridges on the market that unconditionally
cut some LBAs off connected media. This could be a problem
for pre-partitioned drives so GEOM complains and does not create
devices in /dev for slices/partitions preventing access to existing data.

We have kern.geom.part.check_integrity that allows us to correct
partitioning if changed from default 1 to 0 but it works for MBR only.
If backup copy of GPT is unavailable due to decreases number of LBAs,
kernel still does not give access to partitions and prints to dmesg:

GEOM: md0: corrupt or invalid GPT detected.
GEOM: md0: GPT rejected -- may not be recoverable.

This change makes it work for GPT too, so it created partitions in /dev
and prints to dmesg this instead:

GEOM: md0: the secondary GPT table is corrupt or invalid.
GEOM: md0: using the primary only -- recovery suggested.

Then "gpart recover" re-created backup copy of GPT
and allows further manipulations with partitions.

This change is no-op for default configuration having
kern.geom.part.check_integrity=1

Reported by: Alex Korchmar
MFC after: 3 days.