rsxx: Fixes DLPAR add kernel panic if partition still mounted.

A kernel panic would occur on a DLPAR add if there was a partition
still mounted during the DLPAR remove. This bug fix will allow the
user to unmount the partition and bring the driver back into a
good state after the DLPAR add.

Signed-off-by: Philip J Kelleher <pjk1939@linux.vnet.ibm.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>