sctp: fix kernel panic with ERROR chunk containing too many error causes

If ERROR chunk is received with too many error causes in ESTABLISHED
state, the kernel get panic.

This is because sctp limit the max length of cmds to 14, but while
ERROR chunk is received, one error cause will add around 2 cmds by
sctp_add_cmd_sf(). So many error causes will fill the limit of cmds
and panic.

This patch fixed the problem.

This bug can be test by SCTP Conformance Test Suite
<http://networktest.sourceforge.net/>.

Signed-off-by: Wei Yongjun <yjwei@cn.fujitsu.com>
Signed-off-by: Vlad Yasevich <vladislav.yasevich@hp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>