rsxx: enable error return of rsxx_eeh_save_issued_dmas()

Commit d8d595df introduced a bug where we did not check for a NULL
return from kmalloc(). Make rsxx_eeh_save_issued_dmas() return an
error for that case, and make the callers handle that.

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