net: corrections for security_secid_to_secctx returns

security_secid_to_secctx() returns the size of the new context,
whereas previous versions provided that via a pointer parameter.
Correct the type of the value returned in nfqnl_get_sk_secctx()
and the check for error in netlbl_unlhsh_add(). Add an error
check.

Fixes: 2d470c77 ("lsm: replace context+len with lsm_context")
Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>