net/rds: unpin MR pages with unpin_user_pages_dirty_lock()

The pages backing an RDS memory region are pinned in __rds_rdma_map()
with rds_pin_pages(), which uses pin_user_pages_fast(): each page's
refcount is biased by GUP_PIN_COUNTING_BIAS to account the pin.  The
scatterlist is then handed to the IB transport, and the transport
releases the pages in __rds_ib_teardown_mr() with

	set_page_dirty(page);
	put_page(page);

put_page() drops a single reference instead of removing the pin bias,
so every MR teardown permanently strands the remaining references and
the pages are never freed - a userspace-triggerable memory leak of up
to RDS_MAX_MSG_SIZE per RDS_GET_MR/RDS_GET_MR_FOR_DEST call.

The conversion to the pin API updated the unpin sites in rdma.c but
missed this one on the transport side.  Release the pages with
unpin_user_pages_dirty_lock(), which removes the pin bias and also
dirties the page under the folio lock, closing the truncation race
that a bare set_page_dirty() leaves open.

Dirtying under the folio lock can sleep, which is safe in every path
that reaches __rds_ib_teardown_mr(): the registration-reuse path
(rds_ib_map_frmr()) runs in syscall context, and the pool flush
(rds_ib_unreg_frmr()) runs under pool->flush_lock, a mutex, and
already sleeps in rds_ib_post_inv().  The WARN_ON that guarded the
old irq-context set_page_dirty() case is dropped along with it.

Signed-off-by: Allison Henderson <achender@kernel.org>
Link: https://patch.msgid.link/20260730041629.3512480-5-achender@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/rds/ib_rdma.c

@@ -251,9 +251,7 @@ void __rds_ib_teardown_mr(struct rds_ib_mr *ibmr)
 
 			/* FIXME we need a way to tell a r/w MR
 			 * from a r/o MR */
-			WARN_ON(!page->mapping && irqs_disabled());
-			set_page_dirty(page);
-			put_page(page);
+			unpin_user_pages_dirty_lock(&page, 1, true);
 		}
 		kfree(ibmr->sg);