SharedBuffer: Fix bug in return value of release()

Since the equality operator '==' has higher precedence than the
assignment operator '=', we were assigning 'prev' to the result of
our comparison and not the result of mRefs.fetch_sub().

This means that 'prev' would only receive the values 0 or 1.  In
the cases where fetch_sub() returned 0 or 1, we were happening to
get the correct value.  But if fetch_sub() was greator than 1,
we would return to the user 0, instead of the previous reference
count.

We fix this by properly adding parentheses.  We also adjust the
whitespace a little to hopefully make the groupings of the logic
easier to see.

Change-Id: Ib129798a7076854b9ca4f6385c42edbf4fb75e57