UBI: use __packed instead of __attribute__((packed))

There was an attempt to standartize various "__attribute__" and
other macros in order to have potentially portable and more
consistent code, see commit 82ddcb04.

Note, that commit refers Rober Love's blog post, but the URL
is broken, the valid URL is:
http://blog.rlove.org/2005/10/with-little-help-from-your-compiler.html



Moreover, nowadays checkpatch.pl warns about using
__attribute__((packed)):

"WARNING: __packed is preferred over __attribute__((packed))"

It is not a big deal for UBI to use __packed, so let's do it.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>