ext4: convert number of blocks to clusters properly

commit 810da240f221d64bf90020f25941b05b378186fe upstream.

We're using macro EXT4_B2C() to convert number of blocks to number of
clusters for bigalloc file systems.  However, we should be using
EXT4_NUM_B2C().

Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
[bwh: Backported to 3.2:
 - Adjust context
 - Drop changes in ext4_setup_new_descs() and ext4_calculate_overhead()]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>