[SCSI] iscsi class: Use our own workq instead of common system one.
There is just too much going on through the common workq and
something like a scsi device removal through sysfs affects
how long it will take to recover the transport, mark it as
failed, or shut it down gracefully.
Signed-off-by: Mike Christie <michaelc@cs.wisc.edu> Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>