IB/srp: Fix indirect data buffer rkey endianness

Detected by sparse.

Fixes: commit 330179f2 ("IB/srp: Register the indirect data buffer descriptor")
Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Cc: stable <stable@vger.kernel.org> # v4.3+
Cc: Sagi Grimberg <sagig@mellanox.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Sebastian Parschauer <sebastian.riemer@profitbricks.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Doug Ledford <dledford@redhat.com>