IB/mlx5: revisit -Wmaybe-uninitialized warning

A warning that I thought I had fixed before occasionally comes
back in rare randconfig builds (I found 7 instances in the last
100000 builds, originally it was much more frequent):

drivers/infiniband/hw/mlx5/mr.c: In function 'mlx5_ib_reg_user_mr':
drivers/infiniband/hw/mlx5/mr.c:1229:5: error: 'order' may be used uninitialized in this function [-Werror=maybe-uninitialized]
  if (order <= mr_cache_max_order(dev)) {
     ^
drivers/infiniband/hw/mlx5/mr.c:1247:8: error: 'ncont' may be used uninitialized in this function [-Werror=maybe-uninitialized]
drivers/infiniband/hw/mlx5/mr.c:1247:8: error: 'page_shift' may be used uninitialized in this function [-Werror=maybe-uninitialized]
drivers/infiniband/hw/mlx5/mr.c:1260:2: error: 'npages' may be used uninitialized in this function [-Werror=maybe-uninitialized]

I've looked at all those findings again and noticed that they are all
with CONFIG_INFINIBAND_USER_MEM=n, which means ib_umem_get() returns
an error unconditionally and we never initialize or use those variables.
This triggers a condition in gcc iff mr_umem_get() is partially but not
entirely inlined, which in turn depends on the exact combination of
optimization settings. This is a known problem with gcc, with no
easy solution in the compiler, so this adds another workaround that
should be more reliable than my previous attempt.

Returning an error from mlx5_ib_reg_user_mr() earlier means that we
can completely bypass the logic that caused the warning, the compiler
can now see that the variable is never accessed.

Fixes: 14ab8896f5d9 ("IB/mlx5: avoid bogus -Wmaybe-uninitialized warning")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>

drivers/infiniband/hw/mlx5/mr.c

@@ -1206,6 +1206,9 @@ struct ib_mr *mlx5_ib_reg_user_mr(struct ib_pd *pd, u64 start, u64 length,
 	int err;
 	bool use_umr = true;
 
+	if (!IS_ENABLED(CONFIG_INFINIBAND_USER_MEM))
+		return ERR_PTR(-EINVAL);
+
 	mlx5_ib_dbg(dev, "start 0x%llx, virt_addr 0x%llx, length 0x%llx, access_flags 0x%x\n",
 		    start, virt_addr, length, access_flags);