file lock: reorder struct file_lock to save space on 64 bit builds

Reduce sizeof struct file_lock by 8 on 64 bit builds allowing +1 objects
per slab in the file_lock_cache

Signed-off-by: Richard Kennedy <richard@rsk.demon.co.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>