Fix cleanup around socketpair() failure to copy out FDs.

- Use the fs.File, rather than the vfs.FileDescription, in the VFS1 version.

- Check for a nil fs.File/vfs.FileDescription before calling DecRef, which is
  possible if a racing dup2() or dup3() replaces the file descriptor between
  when it is installed and when it is returned. (This is not possible in Linux
  because Linux separates allocation of a file descriptor from binding an
  allocated file descriptor to a struct file, and dup2/dup3 return EBUSY if
  asked to replace an allocated but unbound file descriptor.)

PiperOrigin-RevId: 306517101

pkg/sentry/syscalls/linux/sys_socket.go

@@ -247,8 +247,9 @@ func SocketPair(t *kernel.Task, args arch.SyscallArguments) (uintptr, *kernel.Sy
 	// Copy the file descriptors out.
 	if _, err := t.CopyOut(socks, fds); err != nil {
 		for _, fd := range fds {
-			_, file := t.FDTable().Remove(fd)
-			file.DecRef()
+			if file, _ := t.FDTable().Remove(fd); file != nil {
+				file.DecRef()
+			}
 		}
 		return 0, nil, err
 	}

pkg/sentry/syscalls/linux/vfs2/socket.go

@@ -250,8 +250,9 @@ func SocketPair(t *kernel.Task, args arch.SyscallArguments) (uintptr, *kernel.Sy
 
 	if _, err := t.CopyOut(addr, fds); err != nil {
 		for _, fd := range fds {
-			_, file := t.FDTable().Remove(fd)
-			file.DecRef()
+			if _, file := t.FDTable().Remove(fd); file != nil {
+				file.DecRef()
+			}
 		}
 		return 0, nil, err
 	}