Btrfs: fix fiemap bugs with delalloc

The Btrfs fiemap code wasn't properly returning delalloc extents,
so applications that trust fiemap to decide if there are holes in the
file see holes instead of delalloc.

This reworks the btrfs fiemap code, adding a get_extent helper that
searches for delalloc ranges and also adding a helper for extent_fiemap
that skips past holes in the file.

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