scsi: iscsi: Add helper to remove a session from the kernel

During qedi shutdown we need to stop the iSCSI layer from sending new nops
as pings and from responding to target ones and make sure there is no
running connection cleanups. Commit d1f2ce77 ("scsi: qedi: Fix host
removal with running sessions") converted the driver to use the libicsi
helper to drive session removal, so the above issues could be handled. The
problem is that during system shutdown iscsid will not be running so when
we try to remove the root session we will hang waiting for userspace to
reply.

Add a helper that will drive the destruction of sessions like these during
system shutdown.

Link: https://lore.kernel.org/r/20220616222738.5722-5-michael.christie@oracle.com


Tested-by: Nilesh Javali <njavali@marvell.com>
Reviewed-by: Nilesh Javali <njavali@marvell.com>
Signed-off-by: Mike Christie <michael.christie@oracle.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>