In response to
    http://bugzilla.kernel.org/show_bug.cgi?id=15565
and the fact this capability has been present
in bsg for some time, add SG_FLAG_Q_AT_TAIL flag. It has
the same binary value as the bsg flag and the define name
is the same apart from the leading "B". The semantics are
the same, namely to override the default queue at head
action of the SCSI midlevel when a low level driver
blocks (i.e. when a LLD returns non-zero to a
queuecommand() ). Tested with scsi_debug.

Changelog
   - add SG_FLAG_Q_AT_TAIL flag to override default
     queue at head semantics of the SCSI midlevel queue.

Signed-off-by: Douglas Gilbert <dgilbert@interlog.com>