arm64/ptrace: Preserve previous registers for short regset write

arm64/ptrace: Preserve previous registers for short regset write

BugLink: http://bugs.launchpad.net/bugs/1660993
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>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>