xfs: Remove the second parameter to xfs_sb_count()

Remove the second parameter to xfs_sb_count() since all callers of
the function set them.

Also, fix the header comment regarding it being called periodically.

Signed-off-by: Chandra Seetharaman <sekharan@us.ibm.com>
Signed-off-by: Alex Elder <aelder@sgi.com>