scsi: core: sysfs: Fix hang when device state is set via sysfs

In the Linux kernel, the following vulnerability has been resolved:

scsi: core: sysfs: Fix hang when device state is set via sysfs

This fixes a regression added with:

commit f0f82e2476f6 ("scsi: core: Fix capacity set to zero after
offlinining device")

The problem is that after iSCSI recovery, iscsid will call into the kernel
to set the dev's state to running, and with that patch we now call
scsi_rescan_device() with the state_mutex held. If the SCSI error handler
thread is just starting to test the device in scsi_send_eh_cmnd() then it's
going to try to grab the state_mutex.

We are then stuck, because when scsi_rescan_device() tries to send its I/O
scsi_queue_rq() calls -> scsi_host_queue_ready() -> scsi_host_in_recovery()
which will return true (the host state is still in recovery) and I/O will
just be requeued. scsi_send_eh_cmnd() will then never be able to grab the
state_mutex to finish error handling.

To prevent the deadlock move the rescan-related code to after we drop the
state_mutex.

This also adds a check for if we are already in the running state. This
prevents extra scans and helps the iscsid case where if the transport class
has already onlined the device during its recovery process then we don't
need userspace to do it again plus possibly block that daemon.