vxlan: Add RCU read-side critical sections in the Tx path

vxlan: Add RCU read-side critical sections in the Tx path

[ Upstream commit 804b09be09f8af4eda5346a72361459ba21fcf1b ]

The Tx path does not run from an RCU read-side critical section which
makes the current lockless accesses to FDB entries invalid. As far as I
am aware, this has not been a problem in practice, but traces will be
generated once we transition the FDB lookup to rhashtable_lookup().

Add rcu_read_{lock,unlock}() around the handling of FDB entries in the
Tx path. Remove the RCU read-side critical section from vxlan_xmit_nh()
as now the function is always called from an RCU read-side critical
section.

Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/20250415121143.345227-2-idosch@nvidia.com
Reviewed-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Stable-dep-of: 1f5d2fd1ca04 ("vxlan: Fix NPD in {arp,neigh}_reduce() when using nexthop objects")
Signed-off-by: Sasha Levin <sashal@kernel.org>