x86: coding style fixes to arch/x86/kernel/signal_64.c

Fix all errors and many warnings reported by checkpatch.pl
without change signal_64.o

arch/x86/kernel/signal_64.o
text    data     bss     dec     hex filename
5143       0       8    5151    141f signal_64.o.after
5143       0       8    5151    141f signal_64.o.before
md5:
e68718092b3641cb27e79e55ce57e3ad  signal_64.o.after
e68718092b3641cb27e79e55ce57e3ad  signal_64.o.before

Signed-off-by: Gustavo F. Padovan <gustavo@las.ic.unicamp.br>
Signed-off-by: Ingo Molnar <mingo@elte.hu>