uml: use barrier() instead of mb()

signals_enabled and pending have requirements on the order in which they are
modified.  This used to be done by declaring them volatile and putting an mb()
where the ordering requirements were in effect.

After getting a better (I hope) understanding of how to do this correctly, the
volatile declarations are gone and the mb()'s replaced by barrier()'s.

One of the mb()'s was deleted because I see no problematic writes that could
be re-ordered past that point.

Signed-off-by: Jeff Dike <jdike@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>