Btrfs: fix preallocation and nodatacow checks in O_DIRECT

The O_DIRECT code wasn't checking for multiple references
on preallocated or nodatacow extents.  This means it
wasn't honoring snapshots properly.

The fix here is to add an explicit check for multiple references
This also fixes the math for selecting the correct disk block,
making sure not to go past the end of the extent.

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