Add extack messages to better communicate invalid configuration to the
user.

Example:

# devlink sb pool set pci/0000:01:00.0 pool 0 size 104857600 thtype dynamic
Error: mlxsw_spectrum: Exceeded shared buffer size.
devlink answers: Invalid argument
Signed-off-by: NIdo Schimmel <idosch@mellanox.com>
Acked-by: NJiri Pirko <jiri@mellanox.com>
Reviewed-by: NPetr Machata <petrm@mellanox.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>