osdc/Objecter: recalculate target_* on every _calc_target call

osdc/Objecter: recalculate target_* on every _calc_target call

Any time we are asked to calculate the target we should apply the
pool tiering parameters.  The previous logic of only doing so when the
target hadn't been calculated didn't make a whole lot of sense, and broke
our update of *pi that is needed to get the correct pg_num for the target
pool.  This didn't really matter for old clusters that take the raw pg,
but for luminous and beyond we need the exact spg_t which requires a
correct pg_num.

Signed-off-by: Sage Weil <sage@redhat.com>