ufs: untangle ubh_...block...(), part 3

ufs: untangle ubh_...block...(), part 3

Pass fragment number instead of a block one.  It's available in all
callers and it makes the logics inside those helpers much simpler.
The bitmap they operate upon is with bit per fragment, block being
an aligned group of 1, 2, 4 or 8 adjacent fragments.  We still
need a switch by the number of fragments in block (== number of
bits to check/set/clear), but finding the byte we need to work
with becomes uniform and that makes the things easier to follow.

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