Before this patch CAM periph drivers called both disk_alloc() and disk_create() same time on periph creation. But they prevented disks from opening until the periph probe completion with cam_periph_hold(). As result, especially if disk misbehaves during the probe, GEOM event thread triggered to taste the disk is blocked on open attempt, potentially for a long time, and is unable to process other events.
This patch moves disk_create() call from periph creation to the end of the probe. To allow disk_create() calls from non-sleepable CAM contexts some of its duties requiring memory allocations are moved either back to disk_alloc() or forward to g_disk_create(), so now disk_alloc() and disk_add_alias() are the only disk methods that requires sleeping. If disk fail during the probe disk_create() may just be skipped, going directly to disk_destroy(). Other method calls during that time are just ignored. Since GEOM may now see the disks after CAM bus scan is already completed, introduce per-periph boot hold functions. Enclosure driver already had such mechanism, so just generalize it.