net: bridge: mcast: don't delete permanent entries when fast leave is enabled

When permanent entries were introduced by the commit below, they were
exempt from timing out and thus igmp leave wouldn't affect them unless
fast leave was enabled on the port which was added before permanent
entries existed. It shouldn't matter if fast leave is enabled or not
if the user added a permanent entry it shouldn't be deleted on igmp
leave.

Before:
$ echo 1 > /sys/class/net/eth4/brport/multicast_fast_leave
$ bridge mdb add dev br0 port eth4 grp 229.1.1.1 permanent
$ bridge mdb show
dev br0 port eth4 grp 229.1.1.1 permanent

< join and leave 229.1.1.1 on eth4 >

$ bridge mdb show
$

After:
$ echo 1 > /sys/class/net/eth4/brport/multicast_fast_leave
$ bridge mdb add dev br0 port eth4 grp 229.1.1.1 permanent
$ bridge mdb show
dev br0 port eth4 grp 229.1.1.1 permanent

< join and leave 229.1.1.1 on eth4 >

$ bridge mdb show
dev br0 port eth4 grp 229.1.1.1 permanent

Fixes: ccb1c31a ("bridge: add flags to distinguish permanent mdb entires")
Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>