Btrfs: make sure we're not using obsolete code in btrfs_get_extent

There's code in btrfs_get_extent that should never be used. This patch turns
a WARN_ON(1) into a BUG(), hoping we can remove the transaction code from
btrfs_get_extent soon.

Signed-off-by: Jan Schmidt <list.btrfs@jan-o-sch.net>