net: dcb: move getapptrust to separate function

This patch fixes a frame size warning, reported by kernel test robot.

>> net/dcb/dcbnl.c:1230:1: warning: the frame size of 1244 bytes is
>> larger than 1024 bytes [-Wframe-larger-than=]

The getapptrust part of dcbnl_ieee_fill is moved to a separate function,
and the selector array is now dynamically allocated, instead of stack
allocated.

Tested on microchip sparx5 driver.

Fixes: 6182d587

 ("net: dcb: add new apptrust attribute")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Daniel Machon <daniel.machon@microchip.com>
Link: https://lore.kernel.org/r/20221114092950.2490451-1-daniel.machon@microchip.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>