uml: flush stdout before forking

I was seeing some really weird behaviour where piping UML's output
somewhere would cause output to get duplicated:

  $ ./vmlinux | head -n 40
  Checking that ptrace can change system call numbers...Core dump limits :
          soft - 0
          hard - NONE
  OK
  Checking syscall emulation patch for ptrace...Core dump limits :
          soft - 0
          hard - NONE
  OK
  Checking advanced syscall emulation patch for ptrace...Core dump limits :
          soft - 0
          hard - NONE
  OK
  Core dump limits :
          soft - 0
          hard - NONE

This is because these tests do a fork() which duplicates the non-empty
stdout buffer, then glibc flushes the duplicated buffer as each child
exits.

A simple workaround is to flush before forking.

Cc: stable@vger.kernel.org
Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
Signed-off-by: Richard Weinberger <richard@nod.at>