splice: stop abusing iov_iter_advance() to flush a pipe

Use pipe_discard_from() explicitly in generic_file_read_iter(); don't bother
with rather non-obvious use of iov_iter_advance() in there.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Christian Brauner (Microsoft) <brauner@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>