[PATCH] fix typo in memory barrier docs

Fix cut'n'paste typo - &a and &b are used in other examples, in this one
the doc uses &u and &v.

Signed-off-by: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
Acked-by: David Howells <dhowells@redhat.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>