flexnbd: Add more madvise() hints, both for mirroring out and normal operation.

This is hopefully going to reduce flexnbd rss

src/client.c

@@ -551,6 +551,12 @@ void* client_serve(void* client_uncast)
 		),
 		"Couldn't open/mmap file %s: %s", client->serve->filename, strerror( errno )
 	);
+
+	FATAL_IF_NEGATIVE(
+		madvise( client->mapped, client->serve->size, MADV_SEQUENTIAL ),
+		SHOW_ERRNO( "Failed to madvise() %s", client->serve->filename )
+	);
+
 	debug( "Opened client file fd %d", client->fileno);
 	debug("client: sending hello");
 	client_send_hello(client);

src/mirror.c

@@ -89,6 +89,11 @@ void mirror_init( struct mirror * mirror, const char * filename )
 		filename
 	);
 
+	FATAL_IF_NEGATIVE(
+		madvise( mirror->mapped, size, MADV_SEQUENTIAL ),
+		SHOW_ERRNO( "Failed to madvise() %s", filename )
+	);
+
 	mirror->dirty_map = bitset_alloc(size, 4096);
 
 }