sparc/syscall: fix syscall_get_arch

Sparc's syscall_get_arch was buggy: it returned the task arch, not the
syscall arch.  This could confuse seccomp and audit.

I don't think this is as bad for seccomp as it looks: sparc's 32-bit and
64-bit syscalls are numbered the same.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: David S. Miller <davem@davemloft.net>
Cc: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>