devlink: Fix devlink parallel commands processing

Commit 870c7ad4 ("devlink: protect devlink->dev by the instance
lock") added devlink instance locking inside a loop that iterates over
all the registered devlink instances on the machine in the pre-doit
phase. This can lead to serialization of devlink commands over
different devlink instances.

For example: While the first devlink instance is executing firmware
flash, all commands to other devlink instances on the machine are
forced to wait until the first devlink finishes.

Therefore, in the pre-doit phase, take the devlink instance lock only
for the devlink instance the command is targeting. Devlink layer is
taking a reference on the devlink instance, ensuring the devlink->dev
pointer is valid. This reference taking was introduced by commit
a3806872 ("devlink: take device reference for devlink object").
Without this commit, it would not be safe to access devlink->dev
lockless.

Fixes: 870c7ad4 ("devlink: protect devlink->dev by the instance lock")
Signed-off-by: Shay Drory <shayd@nvidia.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>