mlxsw: spectrum: Limit number of FDB records per learning session

Up until now a learning session ended whenever the number of queried
records was zero. This turned out to be problematic in situations where
a large number of MACs (48K) had to be processed by the switch driver,
as RTNL mutex is held during the learning session.

Instead, limit the number of FDB records that can be processed in a
session to 64. This means that every time the device is queried for
learning notifications (currently, every 100ms), up to 64 records will
be processed by the switch driver.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>