ufs: take the handling of free block counters into a helper

ufs: take the handling of free block counters into a helper

There are 3 places where those counters (many and varied...) are
adjusted - when we are freeing fragments and get an entire block
freed, when we are freeing blocks and (in opposite direction) when
we are grabbing a block.  The logics is identical (modulo the
sign of adjustment) in all three; better take it into a helper -
less duplication and less clutter in the callers that way.

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