audit: simplify audit_enabled check in audit_watch_log_rule_change()

Check the audit_enabled flag and bail immediately.  This does not change
the functionality, but brings the code format in line with similar
checks in audit_tree_log_remove_rule(), audit_mark_log_rule_change(),
and elsewhere in the audit code.

See: https://github.com/linux-audit/audit-kernel/issues/50



Signed-off-by: Richard Guy Briggs <rgb@redhat.com>
[PM: tweaked subject line]
Signed-off-by: Paul Moore <paul@paul-moore.com>