find_first_set doesn't find the least-significant bit on bigendian machines,
so it is really wrong to use it.

ffs is closer, but takes an 'int' and we have a 'unsigned long'.  So use
ffz(~X) to convert a chunksize into a chunkshift.
Signed-off-by: Neil Brown <neilb@suse.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>