Btrfs: fix btrfs send for inline items and compression

The btrfs send code was assuming the offset of the file item into the
extent translated to bytes on disk.  If we're compressed, this isn't
true, and so it was off into extents owned by other files.

It was also improperly handling inline extents.  This solves a crash
where we may have gone past the end of the file extent item by not
testing early enough for an inline extent.  It also solves problems
where we have a whole between the end of the inline item and the start
of the full extent.

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