MIPS: ptrace: Fix PTRACE_PEEKUSR requests for 64-bit FGRs

Use 64-bit accesses for 64-bit floating-point general registers with
PTRACE_PEEKUSR, removing the truncation of their upper halves in the
FR=1 mode, caused by commit bbd426f5 ("MIPS: Simplify FP context
access"), which inadvertently switched them to using 32-bit accesses.

The PTRACE_POKEUSR side is fine as it's never been broken and continues
using 64-bit accesses.

Fixes: bbd426f5 ("MIPS: Simplify FP context access")
Signed-off-by: Maciej W. Rozycki <macro@mips.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: linux-mips@linux-mips.org
Cc: <stable@vger.kernel.org> # 3.15+
Patchwork: https://patchwork.linux-mips.org/patch/19334/


Signed-off-by: James Hogan <jhogan@kernel.org>