mptcp: netlink: split mptcp_pm_parse_addr into two functions

Next patch will need to parse MPTCP_PM_ATTR_ADDR attributes and
fill an mptcp_addr_info structure from a different genl command
callback.

To avoid copy-paste, split the existing function to a helper
that does the common part and then call the helper from the
(renamed)mptcp_pm_parse_entry function.

Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>