sctp: Fix compiler warning when CONFIG_DEBUG_SECTION_MISMATCH=y

WARNING: net/sctp/sctp.o(.text+0x72f1): Section mismatch in reference
from the function sctp_net_init() to the function
.init.text:sctp_proc_init()
The function sctp_net_init() references
the function __init sctp_proc_init().
This is often because sctp_net_init lacks a __init
annotation or the annotation of sctp_proc_init is wrong.

And put __net_init after 'int' for sctp_proc_init - as it is done
everywhere else in the sctp-stack.

Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Acked-by: Vlad Yasevich <vyasevich@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>