arm64/ptrace: Preserve previous registers for short regset write

arm64/ptrace: Preserve previous registers for short regset write

commit 9a17b876b573441bfb3387ad55d98bf7184daf9d upstream.

Ensure that if userspace supplies insufficient data to
PTRACE_SETREGSET to fill all the registers, the thread's old
registers are preserved.

Fixes: 478fcb2cdb23 ("arm64: Debugging support")
Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Acked-by: Will Deacon <Will.Deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
[sw0312.kim: backport linux-4.9.y stable commit for better compat ptrace]
Signed-off-by: Seung-Woo Kim <sw0312.kim@samsung.com>
Change-Id: I1de80715bf95f909290dd04546d8cd6eb2413598