um: Fix FD copy size in os_rcv_fd_msg()

[ Upstream commit df447a3b ]

When copying FDs, the copy size should not include the control
message header (cmsghdr). Fix it.

Fixes: 5cde6096 ("um: generalize os_rcv_fd")
Signed-off-by: Tiwei Bie <tiwei.btw@antgroup.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>