BUG/MINOR: stick-table: do not count the lookup itself in table_trackers()

BUG/MINOR: stick-table: do not count the lookup itself in table_trackers()

In 1.8 when adding multi-threading support with commit 819fc6f563
("MEDIUM: threads/stick-tables: handle multithreads on stick tables"),
the entry's ref_cnt was incremented by one on all trackers and accesses,
and smp_fetch_sc_trackers() learned to return one less than the visible
value so as not to count itself in it.

One of them was missed: table_trackers() returns the number of trackers
without subtracting one so it's always one above the real value. Let's
adjust it like other places do.

This can be backported to all versions.