MINOR: xprt/mux: export all *_io_cb functions so that "show fd" resolves them

MINOR: xprt/mux: export all *_io_cb functions so that "show fd" resolves them

In FD dumps it's often very important to figure what upper layer function
is going to be called. Let's export the few I/O callbacks that appear as
tasklet functions so that "show fd" can resolve them instead of printing
a pointer relative to main. For example:

   1028 : st=0x21(R:rA W:Ra) ev=0x01(heopI) [lc] tmask=0x2 umask=0x2 owner=0x7f00b889b200 iocb=0x65b638(sock_conn_iocb) back=0 cflg=0x00001300 fe=recv mux=H2 ctx=0x7f00c8824de0 h2c.st0=FRH .err=0 .maxid=795 .lastid=-1 .flg=0x0000 .nbst=0 .nbcs=0 .fctl_cnt=0 .send_cnt=0 .tree_cnt=0 .orph_cnt=0 .sub=1 .dsi=795 .dbuf=0@(nil)+0/0 .msi=-1 .mbuf=[1..1|32],h=[0@(nil)+0/0],t=[0@(nil)+0/0] xprt=SSL xprt_ctx=0x7f00c86d0750 xctx.st=0 .xprt=RAW .wait.ev=1 .subs=0x7f00c88252e0(ev=1 tl=0x7f00a07d1aa0 tl.calls=1047 tl.ctx=0x7f00c8824de0 tl.fct=h2_io_cb) .sent_early=0 .early_in=0