RDMA/efa: Add AH cache handling on create and destroy AH

On create AH, first check if the AH cache entry already exists and if
so, returns the already stored AH number. If the entry doesn't exist,
the driver creates it and calls the device to create the AH. A per-entry
mutex serializes concurrent device commands on the same AH cache entry,
ensuring only one thread issues the device create while others wait and
reuse the result. If the device create fails, the entry's user count
remains zero so subsequent threads will retry the device create.

On destroy AH, the user count is decremented under the entry mutex. If
it reaches zero, the driver issues the device destroy command. After
the device destroy completes, it removes the entry from the hashtable
and frees it if no other references exist.  If new users arrived during
the destroy, the entry remains in the hashtable for reuse.

Reviewed-by: Firas Jahjah <firasj@amazon.com>
Reviewed-by: Michael Margolin <mrgolin@amazon.com>
Signed-off-by: Yonatan Nachum <ynachum@amazon.com>
Link: https://patch.msgid.link/20260706170008.1039417-3-ynachum@amazon.com
Signed-off-by: Leon Romanovsky <leon@kernel.org>

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