um: Fix FD copy size in os_rcv_fd_msg()

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

Fixes: 5cde609 ("um: generalize os_rcv_fd")
Signed-off-by: Tiwei Bie <tiwei.btw@antgroup.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

Author: btw616

arch/um/os-Linux/file.c

@@ -535,7 +535,7 @@ ssize_t os_rcv_fd_msg(int fd, int *fds, unsigned int n_fds,
 	    cmsg->cmsg_type != SCM_RIGHTS)
 		return n;
 
-	memcpy(fds, CMSG_DATA(cmsg), cmsg->cmsg_len);
+	memcpy(fds, CMSG_DATA(cmsg), cmsg->cmsg_len - CMSG_LEN(0));
 	return n;
 }