devlink: Rely on driver eswitch thread safety instead of devlink

devlink_nl_cmd_eswitch_set_doit() doesn't hold devlink->lock mutex while
invoking driver callback. This is likely due to eswitch mode setting
involves adding/remove devlink ports, health reporters or
other devlink objects for a devlink device.

So it is driver responsiblity to ensure thread safe eswitch state
transition happening via either sriov legacy enablement or via devlink
eswitch set callback.

Therefore, get() callback should also be invoked without holding
devlink->lock mutex.
Vendor driver can use same internal lock which it uses during eswitch
mode set() callback.
This makes get() and set() implimentation symmetric in devlink core and
in vendor drivers.

Hence, remove holding devlink->lock mutex during eswitch get() callback.

Failing to do so results into below deadlock scenario when mlx5_core
driver is improved to handle eswitch mode set critical section invoked
by devlink and sriov sysfs interface in subsequent patch.

devlink_nl_cmd_eswitch_set_doit()
   mlx5_eswitch_mode_set()
     mutex_lock(esw->mode_lock) <- Lock A
     [...]
     register_devlink_port()
       mutex_lock(&devlink->lock); <- lock B

mutex_lock(&devlink->lock); <- lock B
devlink_nl_cmd_eswitch_get_doit()
   mlx5_eswitch_mode_get()
   mutex_lock(esw->mode_lock) <- Lock A

In subsequent patch, mlx5_core driver uses its internal lock during
get() and set() eswitch callbacks.

Other drivers have been inspected which returns either constant during
get operations or reads the value from already allocated structure.
Hence it is safe to remove the lock in get( ) callback and let vendor
driver handle it.

Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Reviewed-by: Mark Bloch <markb@mellanox.com>
Signed-off-by: Parav Pandit <parav@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>