MFC r327996: geom_disk / scsi_da: deny opening write-protected disks for writing

MFC r327996: geom_disk / scsi_da: deny opening write-protected disks for writing

Ths change consists of two parts.

geom_disk: deny opening a disk for writing if it's marked as
write-protected.  A new disk(9) flag is added to mark write protected
disks.  A possible alternative could be to add another parameter to d_open,
so that the open mode could be passed to it and the disk drivers could
make the decision internally, but the flag required less churn.

scsi_da: add a new phase of disk probing to query the all pages mode
sense page.  We can determine if the disk is write protected using bit 7
of the device specific field in the mode parameter header returned by
MODE SENSE.

PR:		224037