sfc: Fix DMA mapping cleanup in case of an error in TSO

commit a7ebd27a upstream.

We need buffer->len to remain valid to work out the correct address to
be unmapped.  We therefore need to clear buffer->len after the unmap
operation.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>