CVE-2026-31438: netfs: Fix kernel BUG in netfs_limit_iter() for ITER_KVEC iterators

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

netfs: Fix kernel BUG in netfslimititer() for ITERKVEC iterators

When a process crashes and the kernel writes a core dump to a 9P filesystem, kernelwrite() creates an ITERKVEC iterator. This iterator reaches netfslimititer() via netfsunbufferedwrite(), which only handles ITERFOLIOQ, ITERBVEC and ITERXARRAY iterator types, hitting the BUG() for any other type.

Fix this by adding netfslimitkvec() following the same pattern as netfslimitbvec(), since both kvec and bvec are simple segment arrays with pointer and length fields. Dispatch it from netfslimititer() when the iterator type is ITERKVEC.