A comment here incorrectly states that "slack_space" is measured in words, not
bytes.  Remove the comment, and adjust a variable name and a few comments to
clarify the situation.

This is pure cleanup; there should be no change in functionality.
Signed-off-by: NJ. Bruce Fields <bfields@citi.umich.edu>
Signed-off-by: NNeil Brown <neilb@suse.de>
Signed-off-by: NAndrew Morton <akpm@osdl.org>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>