sctp: use the passed in gfp flags instead GFP_KERNEL

This patch doesn't change how the code works because in the current
kernel gfp is always GFP_KERNEL.  But gfp was obviously intended
instead of GFP_KERNEL.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>