x86/ptrace: make genregs[32]_get/set more robust

The loop condition is fragile: we compare an unsigned value to zero, and
then decrement it by something larger than one in the loop.  All the
callers should be passing in appropriately aligned buffer lengths, but
it's better to just not rely on it, and have some appropriate defensive
loop limits.

Acked-by: Roland McGrath <roland@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>