Lock iir(4) and mark it MPSAFE.

• Add a per-device mutex to the softc and use it for bus_dma tags, CAM SIMs, callouts, and interrupt handler.
• Switch from timeout(9) to callout(9).
• Add a separate global mutex to protect the global event buffer ring.
• Return completed index from iir_intr_locked() and remove the global gdt_wait_* variables.
• Remove global list of gdt softcs and replace its use with devclass_get_device().
• Use si_drv1 to store softc pointer in the SDEV_PER_HBA case instead of minor numbers.
• Do math on osreldate instead of dubious char math on osrelease[] that didn't work on 10.0+.
• Use bus_*() instead of bus_space_*().
• Use device_printf() instead of printf() with a unit number.

Tested by: no one