shmem写folio封装 · v1

提供shmem_write_folio包装函数，简化shmem中写入folio的操作接口。

本次提交拆成 7 个子 patch：shmem: provide a shmem_write_folio wrapper、mm/swap: introduce struct swap_io_ctx、mm/swap: also use struct swap_iocb for block I/O、mm/swap: remove count_swpout_vm_event、mm/swap: use swap_ops to register swap device’s methods 等共 7 项改动。