net: netfilter: support for multiple MASQUERADE

This change allow support for same multiple MASQUERADE rule.
without this change if we add multiple MASQUERADE rule then BUG_ON
was causing kernel panic. Replaced BUG_ON to WARN_ON_ONCE to give
only one time warning

Change-Id: I25852e60a5ea2962b5d98c3150eace00db554651
Signed-off-by: Sridhar Ancha <sancha@codeaurora.org>