sctp: fix compile warnings in sctp_tsnmap_num_gabs

net/sctp/tsnmap.c: In function ‘sctp_tsnmap_num_gabs’:
net/sctp/tsnmap.c:347: warning: ‘start’ may be used uninitialized in this function
net/sctp/tsnmap.c:347: warning: ‘end’ may be used uninitialized in this function

Signed-off-by: Shan Wei <shanwei@cn.fujitsu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>