RDMA/cxgb4: add null-ptr-check after ip_dev_find()

RDMA/cxgb4: add null-ptr-check after ip_dev_find()

ip_dev_find() may return NULL and assign it to pdev which is
dereferenced later.
Fix this by checking the return value of ip_dev_find() for NULL
similar to the way it is done with other instances of said function.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Fixes: 1cab775c3e75 ("RDMA/cxgb4: Fix LE hash collision bug for passive open connection")
Signed-off-by: Nikita Zhandarovich <n.zhandarovich@fintech.ru>
Link: https://lore.kernel.org/r/20230201172103.17261-1-n.zhandarovich@fintech.ru
Signed-off-by: Leon Romanovsky <leon@kernel.org>