The Poll fallback (used when IFF_VNET_HDR can't be enabled) has the
same unsafe-pointer-via-uintptr pattern as Offload.rawWrite:
readOne/writeOne build a [2]syscall.Iovec on the stack, pass it to
syscall.Syscall as uintptr, and the kernel then DMAs in/out of the
to/from slices whose Base pointers the iovec holds.
Escape analysis can't see that use, so under GC pressure the
backing memory could be collected or moved mid-syscall.
Add runtime.KeepAlive on the iovec and the user buffer around both
the SYS_READV and SYS_WRITEV syscalls. Same pattern and rationale
as the prior commit on the offload path.
