net: af_unix: implement splice for stream af_unix sockets

unix_stream_recvmsg is refactored to unix_stream_read_generic in this
patch and enhanced to deal with pipe splicing. The refactoring is
inneglible, we mostly have to deal with a non-existing struct msghdr
argument.

Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

fs/splice.c

diff --git a/fs/splice.c b/fs/splice.c
index bfe62ae..4f355a1 100644
--- a/fs/splice.c
+++ b/fs/splice.c
@@ -261,6 +261,7 @@
 
 	return ret;
 }
+EXPORT_SYMBOL_GPL(splice_to_pipe);
 
 void spd_release_page(struct splice_pipe_desc *spd, unsigned int i)
 {