selftests: mlxsw: Fix test failure on Spectrum-4

Remove assumptions about shared buffer cell size and instead query the
cell size from devlink. Adjust the test to send small packets that fit
inside a single cell.

Tested on Spectrum-{1,2,3,4}.

Fixes: 4735402173e6 ("mlxsw: spectrum: Extend to support Spectrum-4 ASIC")
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://lore.kernel.org/r/f7dfbf3c4d1cb23838d9eb99bab09afaa320c4ca.1692268427.git.petrm@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

mlxsw: spectrum: Reduce priority of locally delivered packets

To align with recent recommended values. Will be configurable by future
patches.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

selftests: mlxsw: Add shared buffer traffic test

Test the max shared buffer occupancy for port's pool and port's TC's (using
different types of packets).

Signed-off-by: Shalom Toledo <shalomt@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>