BUG/MINOR: mux-h1: Be sure to swap H1C to splice mode when rcv_pipe() is called

BUG/MINOR: mux-h1: Be sure to swap H1C to splice mode when rcv_pipe() is called

The splicing does not work anymore because the H1 connection is not swap to
splice mode when rcv_pipe() callback function is called. It is important to
set H1C_F_WANT_SPLICE flag to inhibit data receipt via the buffer
API. Otherwise, because there are always data in the buffer, it is not
possible to use the kernel splicing.

This bug was introduced by the commit 2b861bf72 ("MINOR: mux-h1: clean up
conditions to enabled and disabled splicing").

The patch must be backported to 2.4.