9p: Add splice_read wrapper

9p: Add splice_read wrapper

Add a splice_read wrapper for 9p.  We should use copy_splice_read() if
9PL_DIRECT is set and filemap_splice_read() otherwise.  Note that this
doesn't seem to be particularly related to O_DIRECT.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Christoph Hellwig <hch@lst.de>
cc: Al Viro <viro@zeniv.linux.org.uk>
cc: Jens Axboe <axboe@kernel.dk>
cc: Dominique Martinet <asmadeus@codewreck.org>
cc: Eric Van Hensbergen <ericvh@kernel.org>
cc: Latchesar Ionkov <lucho@ionkov.net>
cc: Christian Schoenebeck <linux_oss@crudebyte.com>
cc: v9fs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org
cc: linux-block@vger.kernel.org
cc: linux-mm@kvack.org
Link: https://lore.kernel.org/r/20230522135018.2742245-15-dhowells@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>