msm-ipa: fix use-after-free of rt_tbl

Fix use-after-free of rt_tbl in __ipa_del_flt_rule
by checking if the rt_tbl is already freed.

Change-Id: I09541f65f474dc42f262c603d99f6bbcbb0ce8ec
Signed-off-by: Muralidharan M <murm@codeaurora.org>