mlxsw: spectrum_fid: Use 'refcount_t' for FID reference counting

'refcount_t' is very useful for catching over/under flows. Convert the
FID (Filtering Identifier) objects to use it instead of 'unsigned int'
for their reference count.

A subsequent patch in the series will change the way VXLAN devices hold
/ release the FID reference, which is why the conversion is made now.

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