While posting WRB the next_pointer of the current WRB should point
to itself and the previous WRB next_pointer should point to the
current WRB.

The next pointer value was retrieved during alloc_pdu and was updated
in wrb before ringing the doorbell. The fix retrieves the
next_pointer just before ringing the doorbell and updates in the WRB.
Signed-off-by: John Soni Jose <sony.john@avagotech.com>
Reviewed-by: Mike Christie <michaelc@cs.wisc.edu>
Signed-off-by: James Bottomley <JBottomley@Odin.com>