x86: signal: use signal_fault() in sys_sigreturn()

Impact: cleanup

Call signal_fault() in error route of sys_sigreturn().
Change log level to KERN_EMERG if current is init.

Signed-off-by: Hiroshi Shimamoto <h-shimamoto@ct.jp.nec.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>