inet: frags: enum the flag definitions and add descriptions

Move the flags to an enum definion, swap FIRST_IN/LAST_IN to be in increasing
order and add comments explaining each flag and the inet_frag_queue struct
members.

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