ext4: cleanup to use ext4_grp_offs_to_block()

More cleanup to convert open-coded calculations of the first block
number of a free extent to use ext4_grp_offs_to_block() instead.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Cc: Andreas Dilger <adilger@sun.com>