net/mlx5: HWS, denote how refcounts are protected

Some HWS structs have refcounts that are just u32.
Comment how they are protected and add '__must_hold()'
annotation where applicable.

Signed-off-by: Yevgeny Kliteynik <kliteyn@nvidia.com>
Reviewed-by: Erez Shitrit <erezsh@nvidia.com>
Reviewed-by: Mark Bloch <mbloch@nvidia.com>
Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Link: https://patch.msgid.link/20250102181415.1477316-4-tariqt@nvidia.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>