Btrfs: always pin metadata in discard mode

We have an optimization in btrfs to allow blocks to be
immediately freed if they were allocated in this transaction and never
written.  Otherwise they are pinned and freed when the transaction
commits.

This isn't optimal for discard mode because immediately freeing
them means immediately discarding them.  It is better to give the
block to the pinning code and letting the (slow) discard happen later.

Signed-off-by: Chris Mason <chris.mason@oracle.com>