btrfs: centralize setting REQ_META

Set REQ_META in btrfs_submit_metadata_bio instead of the various callers.
We'll start relying on this flag inside of btrfs in a bit, and this
ensures it is always set correctly.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>