Update extend_data() to take current_size as DataBlocks
Update extend_data() to take current_size as DataBlocks

Since extension of the data region should be done as a multiple of
the DataBlock Size - add a comment as a reminder.

Signed-off-by: Todd Gill <7afbb5ba98932b8ffb9cc9e6675c4792db142529@redhat.com>

stratis-storage/stratisd,mulkieran/stratisd,trgill/stratisd,trgill/stratisd,stratis-storage/stratisd,mulkieran/stratisd,stratis-storage/stratisd