RDMA/uverbs: Tidy input validation of ib_uverbs_rereg_mr()

Unknown flags should be EOPNOTSUPP, only zero flags is EINVAL. Flags is
actually the rereg action to perform.

The checking of the start/hca_va/etc is also redundant and ib_umem_get()
does these checks and returns proper error codes.

Fixes: 7e6edb9b ("IB/core: Add user MR re-registration support")
Link: https://lore.kernel.org/r/20201130075839.278575-2-leon@kernel.orgSigned-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>

drivers/infiniband/core/uverbs_cmd.c

@@ -783,13 +783,15 @@ static int ib_uverbs_rereg_mr(struct uverbs_attr_bundle *attrs)
 	if (ret)
 		return ret;
 
-	if (cmd.flags & ~IB_MR_REREG_SUPPORTED || !cmd.flags)
+	if (!cmd.flags)
 		return -EINVAL;
 
+	if (cmd.flags & ~IB_MR_REREG_SUPPORTED)
+		return -EOPNOTSUPP;
+
 	if ((cmd.flags & IB_MR_REREG_TRANS) &&
-	    (!cmd.start || !cmd.hca_va || 0 >= cmd.length ||
-	     (cmd.start & ~PAGE_MASK) != (cmd.hca_va & ~PAGE_MASK)))
-			return -EINVAL;
+	    (cmd.start & ~PAGE_MASK) != (cmd.hca_va & ~PAGE_MASK))
+		return -EINVAL;
 
 	uobj = uobj_get_write(UVERBS_OBJECT_MR, cmd.mr_handle, attrs);
 	if (IS_ERR(uobj))