We need to convert from bytes to blocks, and we also need to
handle the fact that old smbd's will round up the disk usage
bytecount to even multiples of 1MB, and gracefully fall back 
on an estimate based on the file size instead in that case.

Acked by Samuel Thibault who noted the problem in the first
place, and Jeremy Allison wrt smbd behavior.
Signed-off-by: Linus Torvalds <torvalds@osdl.org>