MFC r274786: Log errors for absent LUNs too.
MFC r274789: Reduce race between LUN destruction and request arrival.
MFC r274790: Remove bunch of unused lun variables.
MFC r274840, r274940:
Make iSCSI frontend less chatty while waiting for tasks termination.
MFC r274756:
Remove residual xpt_release_device() call left after r272406 cleanup.

Excessive release here could trigger use-after-free condition and kernel
panic on LUN 0 disconnect.