dpaa_eth: fix DMA mapping leak

On the error path some fragments remain DMA mapped. Adding a fix
that unmaps all the fragments. Rework cleanup path to be simpler.

Fixes: 8151ee88 ("dpaa_eth: use page backed rx buffers")
Signed-off-by: Madalin Bucur <madalin.bucur@oss.nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>