kernel/exit.c: don't include unused userfaultfd_k.h

Commit dd0db88d8094 ("userfaultfd: non-cooperative: rollback
userfaultfd_exit") removed userfaultfd callback from exit() which makes
the include of <linux/userfaultfd_k.h> unnecessary.

Link: http://lkml.kernel.org/r/1494930907-3060-1-git-send-email-rppt@linux.vnet.ibm.com
Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/exit.c

@@ -51,7 +51,6 @@
 #include <linux/task_io_accounting_ops.h>
 #include <linux/tracehook.h>
 #include <linux/fs_struct.h>
-#include <linux/userfaultfd_k.h>
 #include <linux/init_task.h>
 #include <linux/perf_event.h>
 #include <trace/events/sched.h>