ext4: miscellaneous partial cluster cleanups

Add some casts and rearrange a few statements for improved readability.
Some code can also be simplified and made more readable if we set
partial_cluster to 0 rather than to a negative value when we can tell
we've hit the left edge of the punched region.

Signed-off-by: Eric Whitney <enwlinux@gmail.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>