IB/hfi1: Fix potential use-after-free in PIO and SDMA map teardown

IB/hfi1: Fix potential use-after-free in PIO and SDMA map teardown

The current teardown logic for dd->pio_map and dd->sdma_map frees the
structures while they might still be accessed by RCU readers. Although the
pointer is nulled under a spinlock, the memory is reclaimed before waiting
for the grace period to end.

This patch fixes the sequence by:
1. Extracting the pointer under the lock.
2. Clearing the RCU-protected pointer.
3. Waiting for readers to finish with synchronize_rcu().
4. Finally freeing the memory.

Fixes: 7724105686e7 ("IB/hfi1: add driver files")
Link: https://patch.msgid.link/r/20260206050836.5890-1-lirongqing@baidu.com
Signed-off-by: Li RongQing <lirongqing@baidu.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>