IB/multicast: Check ib_find_pkey() return value

IB/multicast: Check ib_find_pkey() return value

[ Upstream commit d3a2418ee36a59bc02e9d454723f3175dcf4bfd9 ]

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

Fixes: commit 547af76521b3 ("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>
Signed-off-by: Sasha Levin <alexander.levin@verizon.com>