mm, writeback: prevent race when calculating dirty limits

Setting vm_dirty_bytes and dirty_background_bytes is not protected by
any serialization.

Therefore, it's possible for either variable to change value after the
test in global_dirty_limits() to determine whether available_memory
needs to be initialized or not.

Always ensure that available_memory is properly initialized.

Signed-off-by: David Rientjes <rientjes@google.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Michal Hocko <mhocko@suse.cz>
Cc: Rik van Riel <riel@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>