BUG/MINOR: mux-fcgi: Use relative offset to compute contig data in demux buf

BUG/MINOR: mux-fcgi: Use relative offset to compute contig data in demux buf

b_contig_data() should be called with a head-relative offset (0 for the
beginning of readable data). However, in the FCGI multiplexer, to get
contiguous data available in the demux buffer, it is called with
b_head_ofs(dbuf) which returns an absolute buffer position (b->head). So
b->head is counted twice. Because of this bug, the demux buffer could be
realigned while it should not and conversely.

Instead, the offset 0 must be used. So let's fix it.

This patch must be backported as far as 2.4.