net: mvneta: fix possible memory leak in mvneta_swbm_add_rx_fragment

Recycle the page running page_pool_put_full_page() in
mvneta_swbm_add_rx_fragment routine when the last descriptor
contains just the FCS or if the received packet contains more than
MAX_SKB_FRAGS fragments

Fixes: ca0e0146

 ("net: mvneta: move skb build after descriptors processing")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Link: https://lore.kernel.org/r/df6a2bad70323ee58d3901491ada31c1ca2a40b9.1605291228.git.lorenzo@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>