firewire: fw-sbp2: (try to) avoid I/O errors during reconnect

While fw-sbp2 takes the necessary time to reconnect to a logical unit
after bus reset, the SCSI core keeps sending new commands.  They are all
immediately completed with host busy status, and application clients or
filesystems will break quickly.  The SCSI device might even be taken
offline:  http://bugzilla.kernel.org/show_bug.cgi?id=9734

The only remedy seems to be to block the SCSI device until reconnect.
Alas the SCSI core has no useful API to block only one logical unit i.e.
the scsi_device, therefore we block the entire Scsi_Host.  This
currently corresponds to an SBP-2 target.  In case of targets with
multiple logical units, we need to satisfy the dependencies between
logical units by carefully tracking the blocking state of the target and
its units.  We block all logical units of a target as soon as one of
them needs to be blocked, and keep them blocked until all of them are
ready to be unblocked.

Furthermore, as the history of the old sbp2 driver has shown, the
scsi_block_requests() API is a minefield with high potential of
deadlocks.  We therefore take extra measures to keep logical units
unblocked during __scsi_add_device() and during shutdown.

This avoids I/O errors during reconnect in many but alas not in all
cases.  There may still be errors after a re-login had to be performed.
Also, some bridges have been seen to cease fetching management ORBs if
I/O went on up until a bus reset.  In these cases, all management ORBs
time out after mgt_orb_timeout.  The old sbp2 driver is less vulnerable
or maybe not vulnerable to this, for as yet unknown reasons.

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>