i40e/i40evf: Break up xmit_descriptor_count from maybe_stop_tx

i40e/i40evf: Break up xmit_descriptor_count from maybe_stop_tx

Orabug: 22342532

In an upcoming patch I would like to have access to the descriptor count
used for the data portion of the frame.  For this reason I am splitting up
the descriptor count function from the function that stops the ring.

Also in order to try and reduce unnecessary duplication of code I am moving
the slow-path portions of the code out of being inline calls so that we can
just jump to them and process them instead of having to build them into
each function that calls them.

Signed-off-by: Alexander Duyck <aduyck@mirantis.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
(cherry picked from commit 4ec441df25a686518fb369086e2b34a1cedaa6c9)
Signed-off-by: Brian Maly <brian.maly@oracle.com>