ufs_clusteracct(): switch to passing fragment number

ufs_clusteracct(): switch to passing fragment number

Currently all callers pass it a block number.  All of them have it derived
from a fragment number (both fragment and block numbers are within a cylinder
group, and thus 32bit).  Pass it the fragment number instead; none of the
callers has other uses for the block number, so that ends up with cleaner
code.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>