[SCSI] iscsi class: modify handling of replacement timeout

This patch modifies the replacement/recovery_timeout so it works
more like the fc fast io fail tmo.

If userspace tries to set the replacement/recovery_timeout to less than
zero, we will turn off the forced recovery cleanup.

If userspace sets the value to 0 then we will force the recovery
cleanup immediately.

Signed-off-by: Mike Christie <michaelc@cs.wisc.edu>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>