net: dsa: mv88e6xxx: Add FID map cache

Add a cached FID bitmap. This mitigates the need to walk all VTU entries
to find the next free FID.

When flushing the VTU (during init), zero the FID bitmap. Use and
manipulate this bitmap from now on, instead of reading HW for the FID
map.

The repeated VTU walks are costly and can take ~40 mins if ~4000 vlans
are added. Caching the FID map reduces this time to <2 mins.

Signed-off-by: Aryan Srivastava <aryan.srivastava@alliedtelesis.co.nz>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/20241006212905.3142976-1-aryan.srivastava@alliedtelesis.co.nz


Signed-off-by: Jakub Kicinski <kuba@kernel.org>