Fix corners in writepage and btrfs_truncate_page

The extent_io writepage calls needed an extra check for discarding
pages that started on th last byte in the file.

btrfs_truncate_page needed checks to make sure the page was still part
of the file after reading it, and most importantly, needed to wait for
all IO to the page to finish before freeing the corresponding extents on
disk.

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