fs: 9p: add generic splice_write file operation

fs: 9p: add generic splice_write file operation

The default splice operations got removed recently, add it back to 9p
with iter_file_splice_write like many other filesystems do.

Link: http://lkml.kernel.org/r/1606837496-21717-1-git-send-email-asmadeus@codewreck.org
Fixes: 36e2c7421f02 ("fs: don't allow splice read/write without explicit ops")
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>
Acked-by: Toke Høiland-Jørgensen <toke@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>