mlxsw: spectrum_buffers: Limit size of pools

The shared buffer pools are containers whose size is used to calculate
the maximum usage for packets from / to a specific port / {port, PG/TC},
when dynamic threshold is employed.

While it's perfectly fine for the sum of the pools to exceed the maximum
size of the shared buffer, a single pool cannot.

Add a check when the pool size is set and forbid sizes larger than the
maximum size of the shared buffer.

Without the patch:
$ devlink sb pool set pci/0000:03:00.0 pool 0 size 999999999 thtype
dynamic
// No error is returned

With the patch:
$ devlink sb pool set pci/0000:03:00.0 pool 0 size 999999999 thtype
dynamic
devlink answers: Invalid argument

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>