net: sctp: sm_make_chunk.c: delete duplicated words + fix typo

Drop the repeated words "for", "that", and "a".
Change "his" to "this".

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Vlad Yasevich <vyasevich@gmail.com>
Cc: Neil Horman <nhorman@tuxdriver.com>
Cc: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Cc: linux-sctp@vger.kernel.org
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>