sfc: handle error pointers returned by rhashtable_lookup_get_insert_fast()

Several places in TC offload code assumed that the return from
 rhashtable_lookup_get_insert_fast() was always either NULL or a valid
 pointer to an existing entry, but in fact that function can return an
 error pointer.  In that case, perform the usual cleanup of the newly
 created entry, then pass up the error, rather than attempting to take a
 reference on the old entry.

Fixes: d902e1a7

 ("sfc: bare bones TC offload on EF100")
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Edward Cree <ecree.xilinx@gmail.com>
Link: https://lore.kernel.org/r/20230919183949.59392-1-edward.cree@amd.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>