netns: returns always an id in __peernet2id()

netns: returns always an id in __peernet2id()

All callers of this function expect a nsid, not an error.
Thus, returns NETNSA_NSID_NOT_ASSIGNED in case of error so that callers
don't have to convert the error to NETNSA_NSID_NOT_ASSIGNED.

Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Acked-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>
Orabug: 23634951
(cherry picked from commit 109582af18b9aade9385ea6609a792f80a7d70ca)
Signed-off-by: Cathy Zhou <Cathy.Zhou@Oracle.COM>
Reviewed-by: Jack Vogel <jack.vogel@oracle.com>
Signed-off-by: Dhaval Giani <dhaval.giani@oracle.com>