xen-netback: Allocate fraglist early to avoid complex rollback

Determine if a fraglist is needed in the tx path, and allocate it if
necessary before setting up the copy and map operations.
Otherwise, undoing the copy and map operations is tricky.

This fixes a use-after-free: if allocating the fraglist failed, the copy
and map operations that had been set up were still executed, writing
over the data area of a freed skb.

Signed-off-by: Ross Lagerwall <ross.lagerwall@citrix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>