xfs: nothing special about 1-block log sector

There are a number of places where a log sector size of 1 uses
special case code.  The round_up() and round_down() macros
produce the correct result even when the log sector size is 1, and
this eliminates the need for treating this as a special case.

Signed-off-by: Alex Elder <aelder@sgi.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>