RDMA/mlx5: Add missing store/release for lock elision pattern

RDMA/mlx5: Add missing store/release for lock elision pattern

mlx5 has a common pattern implementing a device-global singleton resource
where it checks the resource pointer for !NULL and then skips obtaining
the lock.

This is not ordered properly as observing !NULL doesn't mean that all the
data under that pointer is also visible on this CPU when the lock is not
taken.

Use a release/acquire pairing to explicitly manage this.

Pointed out by sashiko, Codex found more cases.

Fixes: 5895e70f2e6e ("IB/mlx5: Allocate resources just before first QP/SRQ is created")
Fixes: 638420115cc4 ("IB/mlx5: Create UMR QP just before first reg_mr occurs")
Link: https://sashiko.dev/#/patchset/SYBPR01MB7881E1E0970268BD69C0BA75AF2B2%40SYBPR01MB7881.ausprd01.prod.outlook.com
Link: https://patch.msgid.link/r/3-v1-41f3135e5565+9d2-rdma_ai_fixes1_jgg@nvidia.com
Assisted-by: Codex:GPT-5.5
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>