splice: Make do_splice_to() generic and export it

Rename do_splice_to() to vfs_splice_read() and export it so that it can be
used as a helper when calling down to a lower layer filesystem as it
performs all the necessary checks[1].

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Christian Brauner <brauner@kernel.org>
cc: Miklos Szeredi <miklos@szeredi.hu>
cc: Jens Axboe <axboe@kernel.dk>
cc: Al Viro <viro@zeniv.linux.org.uk>
cc: John Hubbard <jhubbard@nvidia.com>
cc: David Hildenbrand <david@redhat.com>
cc: Matthew Wilcox <willy@infradead.org>
cc: linux-unionfs@vger.kernel.org
cc: linux-block@vger.kernel.org
cc: linux-fsdevel@vger.kernel.org
cc: linux-mm@kvack.org
Link: https://lore.kernel.org/r/CAJfpeguGksS3sCigmRi9hJdUec8qtM9f+_9jC1rJhsXT+dV01w@mail.gmail.com/ [1]
Link: https://lore.kernel.org/r/20230522135018.2742245-6-dhowells@redhat.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>