net: ipmr: remove unused MFC_NOTIFY flag and make the flags enum

MFC_NOTIFY was introduced in kernel 2.1.68 but afaik it hasn't been used
and I couldn't find any users currently so just remove it. Only
MFC_STATIC is left, so move it into an enum, add a description and use
BIT().

Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>