ptrace: cleanup arch_ptrace() on sparc

Factor out struct fps and remove redundant castings.

Signed-off-by: Namhyung Kim <namhyung@gmail.com>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>