sparc64: Fix sparc64_set_context stack handling.

[ Upstream commit 397d1533b6cce0ccb5379542e2e6d079f6936c46 ]

Like a signal return, we should use synchronize_user_stack() rather
than flush_user_windows().

Reported-by: Ilya Malakhov <ilmalakhovthefirst@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>