ipmr_base: Do not reset index in mr_table_dump

e is the counter used to save the location of a dump when an
skb is filled. Once the walk of the table is complete, mr_table_dump
needs to return without resetting that index to 0. Dump of a specific
table is looping because of the reset because there is no way to
indicate the walk of the table is done.

Move the reset to the caller so the dump of each table starts at 0,
but the loop counter is maintained if a dump fills an skb.

Fixes: e1cedae1 ("ipmr: Refactor mr_rtm_dumproute")
Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>