btrfs: refactor priority_reclaim_metadata_space

With the eviction flushing stuff we'll want to allow for different
states, but still work basically the same way that
priority_reclaim_metadata_space works currently.  Refactor this to take
the flushing states and size as an argument so we can use the same logic
for limit flushing and eviction flushing.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>