Btrfs: fix check-integrity to look at the referenced data properly

We were looking at file_extent_num_bytes unconditionally when looking at
referenced data bytes, but this isn't correct for compression.  Fix this by
checking the compression of the file extent we are and setting num_bytes to
disk_num_bytes in the case of compression so that we are marking the proper
bytes as referenced.  This fixes check_int_data freaking out when running
btrfs/004.  Thanks,

Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <clm@fb.com>