net: sctp: fix warning "NULL check before some freeing functions is not needed"

This patch removes NULL checks before calling kfree.

fixes below issues reported by coccicheck
net/sctp/sm_make_chunk.c:2586:3-8: WARNING: NULL check before some
freeing functions is not needed.
net/sctp/sm_make_chunk.c:2652:3-8: WARNING: NULL check before some
freeing functions is not needed.
net/sctp/sm_make_chunk.c:2667:3-8: WARNING: NULL check before some
freeing functions is not needed.
net/sctp/sm_make_chunk.c:2684:3-8: WARNING: NULL check before some
freeing functions is not needed.

Signed-off-by: Hariprasad Kelam <hariprasad.kelam@gmail.com>
Acked-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>