uml: disable winch irq before freeing handler data

Disable the winch irq early to make sure we don't take an interrupt part
way through the freeing of the handler data, resulting in a crash on
shutdown:

  winch_interrupt : read failed, errno = 9
  fd 13 is losing SIGWINCH support
  ------------[ cut here ]------------
  WARNING: at lib/list_debug.c:48 list_del+0xc6/0x100()
  list_del corruption, next is LIST_POISON1 (00100100)
  082578c8:  [<081fd77f>] dump_stack+0x22/0x24
  082578e0:  [<0807a18a>] warn_slowpath_common+0x5a/0x80
  08257908:  [<0807a23e>] warn_slowpath_fmt+0x2e/0x30
  08257920:  [<08172196>] list_del+0xc6/0x100
  08257940:  [<08060244>] free_winch+0x14/0x80
  08257958:  [<080606fb>] winch_interrupt+0xdb/0xe0
  08257978:  [<080a65b5>] handle_IRQ_event+0x35/0xe0
  08257998:  [<080a8717>] handle_edge_irq+0xb7/0x170
  082579bc:  [<08059bc4>] do_IRQ+0x34/0x50
  082579d4:  [<08059e1b>] sigio_handler+0x5b/0x80
  082579ec:  [<0806a374>] sig_handler_common+0x44/0xb0
  08257a68:  [<0806a538>] sig_handler+0x38/0x50
  08257a78:  [<0806a77c>] handle_signal+0x5c/0xa0
  08257a9c:  [<0806be28>] hard_handler+0x18/0x20
  08257aac:  [<00c14400>] 0xc14400

Signed-off-by: Will Newton <will.newton@gmail.com>
Acked-by: WANG Cong <xiyou.wangcong@gmail.com>
Cc: Jeff Dike <jdike@addtoit.com>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>