IB/multicast: Check ib_find_pkey() return value

This patch avoids that Coverity complains about not checking the
ib_find_pkey() return value.

Fixes: commit 547af765 ("IB/multicast: Report errors on multicast groups if P_key changes")
Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Cc: Sean Hefty <sean.hefty@intel.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Doug Ledford <dledford@redhat.com>