mm: fix pdflush thread creation upper bound

Fix a race on creating pdflush threads.  Without the patch, it is possible
to create more than MAX_PDFLUSH_THREADS threads, and this has been
observed in practice on IO loaded SMP machines.

The fix involves moving the lock around to protect the check against the
thread count and correctly dealing with thread creation failure.

This fix also _mostly_ repairs a race condition on how quickly the threads
are created.  The original intent was to create a pdflush thread (up to
the max allowed) every second.  Without this patch is is possible to
create NCPUS pdflush threads concurrently.  The 'mostly' caveat is because
an assumption is made that thread creation will be successful.  If we fail
to create the thread, the miss is not considered fatal.  (we will try
again in 1 second)

Signed-off-by: Peter W Morreale <pmorreale@novell.com>
Reviewed-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>