nat: report correct EI per-user session limit

nat: report correct EI per-user session limit

Type: fix

When enabling the endpoint independent NAT44 plugin, user_sessions
determines the maximum number of translations that can be active for
a single inside address. If 0 is passed in, a default value is used
but 0 is still stored in the field that is used to populate reply
messages to nat44_ei_show_running_config,

At the time of enabling the plugin, if user_sessions is 0, update the
field which is used by nat44_ei_show_running_config to contain the
default per-user limit which gets used by the EI nodes.

Change-Id: I6b060d85bcd42d91db879b95a8b07c6844bcd2a5
Signed-off-by: Matthew Smith <mgsmith@netgate.com>