ibcore: Fix a use-after-free in ucma_resolve_ip().

ibcore: Fix a use-after-free in ucma_resolve_ip().

There is a race condition between ucma_close() and ucma_resolve_ip():

CPU0                            CPU1
ucma_resolve_ip():              ucma_close():

ctx = ucma_get_ctx(file, cmd.id);

        list_for_each_entry_safe(ctx, tmp, &file->ctx_list, list) {
                mutex_lock(&mut);
                idr_remove(&ctx_idr, ctx->id);
                mutex_unlock(&mut);
                ...
                mutex_lock(&mut);
                if (!ctx->closing) {
                        mutex_unlock(&mut);
                        rdma_destroy_id(ctx->cm_id);
                ...
                ucma_free_ctx(ctx);
        }

ret = rdma_resolve_addr();
ucma_put_ctx(ctx);

Before idr_remove(), ucma_get_ctx() could still find the ctx
and after rdma_destroy_id(), rdma_resolve_addr() may still
access id_priv pointer. Also, ucma_put_ctx() may use ctx after
ucma_free_ctx() too.

ucma_close() should call ucma_put_ctx() too which tests the
refcnt and waits for the last one releasing it. The similar
pattern is already used by ucma_destroy_id().

Linux commit:
5fe23f262e0548ca7f19fb79f89059a60d087d22

MFC after:	1 week
Reviewed by:	kib
Sponsored by:	Mellanox Technologies // NVIDIA Networking