um: Fix possible deadlock in sig_handler_common()

We are in atomic context and must not sleep.
Sleeping here is possible since malloc() maps
to kmalloc() with GFP_KERNEL.

Cc: stable@vger.kernel.org
Fixes: b6024b21

 ("um: extend fpstate to _xstate to support YMM registers")
Signed-off-by: Richard Weinberger <richard@nod.at>