alpha: strncpy/strncat fixes

First of all, thanks to Bob Tracy <rct@frus.com> and
Michael Cree <mcree@orcon.net.nz> for testing.
Especially to Bob, as he has done titanic multi-day git-bisect
work that finally helped to reproduce and nail down the bug
(http://bugzilla.kernel.org/show_bug.cgi?id=9457).

[ev6-]stxncpy.S: it's t12, not t2 register that is supposed to contain
the last byte offset upon return. As a result of wrong register use
(which was my fault back in 2003, IIRC), under some circumstances extra
terminating zero bytes were added to destination string. This particularly
led to incorrect DEVPATH strings generated in uevent and therefore to udev
problems.

strncpy.S: unrelated bug I found while testing the above fix - destination
is not properly zero-padded then a byte count exceeds source length.
Actually this is addition to strncpy fix from last year.

Signed-off-by: Ivan Kokshaysky <ink@jurassic.park.msu.ru>
Cc: Richard Henderson <rth@twiddle.net>
Cc: Bob Tracy <rct@frus.com>
Cc: Michael Cree <mcree@orcon.net.nz>
Cc: Kay Sievers <kay.sievers@vrfy.org>
Cc: "Rafael J. Wysocki" <rjw@sisk.pl>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>