Fix for WITNESS warning while doing xpt_rescan.

Fix for WITNESS warning while doing xpt_rescan.
This happen when converting any JBOD to RAID or creating
any new RAID from Unconfigured Drives.

Without this fix, user may see below call trace if  WITNESS is enabled.

witness_warn() at witness_warn+0x4b5/frame 0xfffffe011f929a00
uma_zalloc_arg() at uma_zalloc_arg+0x3b/frame 0xfffffe011f929a70
malloc() at malloc+0x192/frame 0xfffffe011f929ac0
mrsas_bus_scan_sim() at mrsas_bus_scan_sim+0x32/frame 0xfffffe011f929af0
mrsas_aen_handler() at mrsas_aen_handler+0x11c/frame 0xfffffe011f929b20
taskqueue_run_locked() at taskqueue_run_locked+0xf0/frame 0xfffffe011f929b80
taskqueue_thread_loop() at taskqueue_thread_loop+0x9b/frame 0xfffffe011f929bb0
fork_exit() at fork_exit+0x84/frame 0xfffffe011f929bf0
fork_trampoline() at fork_trampoline+0xe/frame 0xfffffe011f929bf0

Submitted by:   kadesai
Reviewed by:    ambrisko
MFC after:      3 days