metag/ptrace: Preserve previous registers for short regset write

metag/ptrace: Preserve previous registers for short regset write

BugLink: http://bugs.launchpad.net/bugs/1678930
commit a78ce80d2c9178351b34d78fec805140c29c193e upstream.

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

Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Acked-by: James Hogan <james.hogan@imgtec.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>