sctp: drop __packed from almost all SCTP structures

__packed is considered harmful as it potentially generates code that
doesn't perform well and its usage should be avoided as much as
possible.

This patch drops __packed from all SCTP structures except one, which is
sctp_signed_cookie. In there it's required, as per changelog on
commit 9834a2bb ("[SCTP]: Fix sctp_cookie alignment in the packet.").

After this patch, no alignment changes neither in x86 or x86_64 and
no exceptions were noticed during testing on both archs.

Code size for SCTP module also didn't change with this patch.

Cc: David Miller <davem@davemloft.net>
Cc: David Laight <David.Laight@ACULAB.COM>
Signed-off-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>