o In iflib_txq_drain():

• Remove desc_used, which is only ever written to.
• Remove a dead store to reclaimed.
• Don't recycle avail.
• Sort variables according to style(9). These changes will make a subsequent commit easier to read.

o In iflib_tx_credits_update(), don't bother checking whether the

ift_txd_credits_update method pointer is NULL; _iflib_pre_assert()
asserts upfront that this method has been assigned and functions
like iflib_{fast_intr_rxtx,netmap_timer_adjust,txq_can_drain}()
and _task_fn_tx() were already unconditionally relying on the
method being callable.

(cherry picked from commit c2c5d1e7874dd2928bc5e85912e59541741f74a3)