reiserfs: clean-up function return type

Turn 'reiserfs_flush_old_commits()' into a void function because the callers
do not cares about what it returns anyway.

We are going to remove the 'sb->s_dirt' field completely and this patch is a
small step towards this direction.

Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>