vxlan: Use a single lock to protect the FDB table

vxlan: Use a single lock to protect the FDB table

Currently, the VXLAN driver stores FDB entries in a hash table with a
fixed number of buckets (256). Subsequent patches are going to convert
this table to rhashtable with a linked list for entry traversal, as
rhashtable is more scalable.

In preparation for this conversion, move from a per-bucket spin lock to
a single spin lock that protects the entire FDB table.

The per-bucket spin locks were introduced by commit fe1e0713bbe8
("vxlan: Use FDB_HASH_SIZE hash_locks to reduce contention") citing
"huge contention when inserting/deleting vxlan_fdbs into the fdb_head".

It is not clear from the commit message which code path was holding the
spin lock for long periods of time, but the obvious suspect is the FDB
cleanup routine (vxlan_cleanup()) that periodically traverses the entire
table in order to delete aged-out entries.

This will be solved by subsequent patches that will convert the FDB
cleanup routine to traverse the linked list of FDB entries using RCU,
only acquiring the spin lock when deleting an aged-out entry.

The change reduces the size of the VXLAN device structure from 3600
bytes to 2576 bytes.

Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/20250415121143.345227-7-idosch@nvidia.com
Reviewed-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>