SUNRPC: Remove resource leak in svc_rdma_send_error()

We leak the memory allocated to 'ctxt' when we return after
'ib_dma_mapping_error()' returns !=0.

Signed-off-by: Jesper Juhl <jj@chaosbits.net>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>