stmmac: fix oversized frame reception

The receive skb buffers can be preallocated when the link is opened
according to mtu size.
While testing on a network environment with not standard MTU (e.g. 3000),
a panic occurred if an incoming packet had a length greater than rx skb
buffer size. This is because the HW is programmed to copy, from the DMA,
an Jumbo frame and the Sw must check if the allocated buffer is enough to
store the frame.

Signed-off-by: Alexandre TORGUE <alexandre.torgue@st.com>
Signed-off-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>