fs/pipe: allow sendfile() to pipe again

After commit 36e2c742 ("fs: don't allow splice read/write
without explicit ops") sendfile() could no longer send data
from a real file to a pipe, breaking for example certain cgit
setups (e.g. when running behind fcgiwrap), because in this
case cgit will try to do exactly this: sendfile() to a pipe.

Fix this by using iter_file_splice_write for the splice_write
method of pipes, as suggested by Christoph.

Cc: stable@vger.kernel.org
Fixes: 36e2c742 ("fs: don't allow splice read/write without explicit ops")
Suggested-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Tested-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>