Btrfs: release path before starting transaction in can_nocow_extent

We can't be holding tree locks while we try to start a transaction, we will
deadlock.  Thanks,

Reported-by: Sage Weil <sage@inktank.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Sage Weil <sage@inktank.com>