sctp: fix shadow symbol in net/sctp/tsnmap.c

net/sctp/tsnmap.c:164:16: warning: symbol '_end' shadows an earlier one
include/asm-generic/sections.h:13:13: originally declared here

Renamed renamed _end to end_ and _start (for consistence).

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