sctp: remove shadowed symbols

Fixes the following sparse warnings:
net/sctp/sm_make_chunk.c:1457:9: warning: symbol 'len' shadows an earlier one
net/sctp/sm_make_chunk.c:1356:23: originally declared here
net/sctp/socket.c:1534:22: warning: symbol 'chunk' shadows an earlier one
net/sctp/socket.c:1387:20: originally declared here

Signed-off-by: Sebastian Siewior <sebastian@breakpoint.cc>
Signed-off-by: Vlad Yasevich <vladislav.yasevich@hp.com>