BUG/MINOR: stick-tables: Fix return value for __stksess_kill()

BUG/MINOR: stick-tables: Fix return value for __stksess_kill()

The commit 9938fb9c7 ("BUG/MEDIUM: stick-tables: Fix race with peers when
killing a sticky session") introduced a regression.

__stksess_kill() must always return 0 if the session cannot be released. But
when the ref_cnt is tested under the update lock, a success is reported if
the session is still in-used. 0 must be returned in that case.

This bug is harmless because callers never use the return value of
__stksess_kill() or stksess_kill().

This bug must be backported as far as 3.0.