exec: don't turn PF_KTHREAD off when a target command was not found

Presently do_execve() turns PF_KTHREAD off before search_binary_handler().
 THis has a theorical risk of PF_KTHREAD getting lost.  We don't have to
turn PF_KTHREAD off in the ENOEXEC case.

This patch moves this flag modification to after the finding of the
executable file.

This is only a theorical issue because kthreads do not call do_execve()
directly.  But fixing would be better.

Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Acked-by: Roland McGrath <roland@redhat.com>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>