cluster - xdisk automatic BIO restart
* The xdisk driver now detects failed transactions due to failed circuits
and will restart the BIOs on another circuit or hold onto them until
connectivity is restored and a new circuit is reforged.
Automatic restarts only occur if the xa* disk is open()'d (i.e. mounted
or being accessed by userland). Kernel disk subsystem probes on attach
will be failed normally and not stall on lost connectivity.
* subr_diskiocom now reports the correct DMSG error code for failed BIOs
instead of reporting a kernel error code.