[NET] back: Initialise first fragment properly

[NET] back: Initialise first fragment properly

The first fragment is used to store the pending_idx of the leading
txreq if it doesn't fit in the head area.  When it does fit into
the head we need to ensure that the first fragment contains a value
that is not equal to pending_idx as that's what we use to distinguish
between the two cases in a a number of places.

This patch sets the first fragment to ~0 which is not equal to any
valid pending_idx.  Without this initialisation, we may double-free
a pending_idx if the first fragment happened to contain a value
equal to it (this usually happened with pending_idx 0).

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>