IB/core: Add might_sleep() annotation to ib_init_ah_from_wc()

For RoCE, ib_init_ah_from_wc() can follow the path

    ib_init_ah_from_wc() ->
      rdma_addr_find_l2_eth_by_grh() ->
        rdma_resolve_ip()

and rdma_resolve_ip() will sleep in kzalloc() and wait_for_completion().

However, developers will not see any warnings if they use ib_init_ah_from_wc()
in an atomic context and test only on IB, because the function doesn't
sleep in that case.

Add a might_sleep() so that lockdep will catch bugs no matter what hardware is
used to test.

Signed-off-by: Roland Dreier <roland@purestorage.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>