kernfs: wire up ->splice_read and ->splice_write

Wire up the splice_read and splice_write methods to the default
helpers using ->read_iter and ->write_iter now that those are
implemented for kernfs.  This restores support to use splice and
sendfile on kernfs files.

Fixes: 36e2c742 ("fs: don't allow splice read/write without explicit ops")
Reported-by: Siddharth Gupta <sidgup@codeaurora.org>
Tested-by: Siddharth Gupta <sidgup@codeaurora.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20210120204631.274206-4-hch@lst.deSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

fs/kernfs/file.c

@@ -949,6 +949,8 @@ const struct file_operations kernfs_file_fops = {
 	.release	= kernfs_fop_release,
 	.poll		= kernfs_fop_poll,
 	.fsync		= noop_fsync,
+	.splice_read	= generic_file_splice_read,
+	.splice_write	= iter_file_splice_write,
 };
 
 /**