net: sctp: Inline the functions from command.c

sctp_init_cmd_seq() and sctp_next_cmd() are only called from one place.
The call sequence for sctp_add_cmd_sf() is likely to be longer than
the inlined code.
With sctp_add_cmd_sf() inlined the compiler can optimise repeated calls.

Signed-off-by: David Laight <david.laight@aculab.com>
Signed-off-by: David S. Miller <davem@davemloft.net>