ixgbe: Fix Tx map failure path

This patch is a partial revert of "ixgbe: Don't bother clearing buffer
memory for descriptor rings". Specifically I messed up the exception
handling path a bit and this resulted in us incorrectly adding the count
back in when we didn't need to.

In order to make this simpler I am reverting most of the exception handling
path change and instead just replacing the bit that was handled by the
unmap_and_free call.

Fixes: ffed21bc

 ("ixgbe: Don't bother clearing buffer memory for descriptor rings")
Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>