cxgb4: fix the panic caused by non smac rewrite

SMT entry is allocated only when loopback Source MAC
rewriting is requested. Accessing SMT entry for non
smac rewrite cases results in kernel panic.

Fix the panic caused by non smac rewrite

Fixes: 937d8420 ("cxgb4: set up filter action after rewrites")
Signed-off-by: Raju Rangoju <rajur@chelsio.com>
Link: https://lore.kernel.org/r/20201118143213.13319-1-rajur@chelsio.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>