CVE-2026-74299: RDMA/core: Fix FRMR aging push to queue error flow

In the Linux kernel, the following vulnerability has been resolved:

RDMA/core: Fix FRMR aging push to queue error flow

Aging pools with pinned handles requires moving handles from the active queue to a non-empty inactive queue that might fail on new page allocation, we are currently not handling the fault and leaking any mkey that fails the push.

Fix by Introducing pushqueuetoqueuelocked() that fills the destination's partial tail page from the source and then splices the remaining source pages onto the destination, performing no allocation.

Replace the per-handle move loop in agepinnedpool() and the open-coded splice in poolagingwork() with calls to the helper. As the helper cannot fail under memory pressure, removing a class of GFPATOMIC allocations under the pool lock and simplifying the error flow.