get rid of the second argument of acct_kill()

Replace the old ns->bacct only with NULL and only if it still points
to acct.  And assign the new value to it *before* calling acct_kill()
in acct_on().  That way we don't need to pass the new acct to acct_kill().

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>