net/mlx5: Bridge, set debugfs access right to root-only

As suggested during code review set the access rights for bridge 'fdb'
debugfs file to root-only.

Fixes: 791eb78285e8 ("net/mlx5: Bridge, expose FDB state via debugfs")
Reported-by: Jakub Kicinski <kuba@kernel.org>
Link: https://lore.kernel.org/netdev/20230619120515.5045132a@kernel.org/
Signed-off-by: Vlad Buslov <vladbu@nvidia.com>
Reviewed-by: Gal Pressman <gal@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>

net/mlx5: Bridge, expose FDB state via debugfs

For debugging purposes expose offloaded FDB state (flags, counters, etc.)
via debugfs inside 'esw' root directory. Example debugfs file output:

$ cat mlx5/0000\:08\:00.0/esw/bridge/bridge1/fdb
DEV MAC VLAN PACKETS BYTES LASTUSE FLAGS
enp8s0f0_1 e4:0a:05:08:00:06 2 2 204 4295567112 0x0
enp8s0f0_0 e4:0a:05:08:00:03 2 3 278 4295567112 0x0

Signed-off-by: Vlad Buslov <vladbu@nvidia.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Reviewed-by: Gal Pressman <gal@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>