io_uring/rw: free potentially allocated iovec on cache put failure

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

io_uring/rw: free potentially allocated iovec on cache put failure

If a read/write request goes through io_req_rw_cleanup() and has an allocated iovec attached and fails to put to the rw_cache, then it may end up with an unaccounted iovec pointer. Have io_rw_recycle() return whether it recycled the request or not, and use that to gauge whether to free a potential iovec or not.