net/smc: fix compile warning for smc_sysctl

kernel test robot reports multiple warning for smc_sysctl:

  In file included from net/smc/smc_sysctl.c:17:
>> net/smc/smc_sysctl.h:23:5: warning: no previous prototype \
	for function 'smc_sysctl_init' [-Wmissing-prototypes]
  int smc_sysctl_init(void)
       ^
and
  >> WARNING: modpost: vmlinux.o(.text+0x12ced2d): Section mismatch \
  in reference from the function smc_sysctl_exit() to the variable
  .init.data:smc_sysctl_ops
  The function smc_sysctl_exit() references
  the variable __initdata smc_sysctl_ops.
  This is often because smc_sysctl_exit lacks a __initdata
  annotation or the annotation of smc_sysctl_ops is wrong.

and
  net/smc/smc_sysctl.c: In function 'smc_sysctl_init_net':
  net/smc/smc_sysctl.c:47:17: error: 'struct netns_smc' has no member named 'smc_hdr'
     47 |         net->smc.smc_hdr = register_net_sysctl(net, "net/smc", table);

Since we don't need global sysctl initialization. To make things
clean and simple, remove the global pernet_operations and
smc_sysctl_{init|exit}. Call smc_sysctl_net_{init|exit} directly
from smc_net_{init|exit}.

Also initialized sysctl_autocorking_size if CONFIG_SYSCTL it not
set, this make sure SMC autocorking is enabled by default if
CONFIG_SYSCTL is not set.

Fixes: 462791bb ("net/smc: add sysctl interface for SMC")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Dust Li <dust.li@linux.alibaba.com>
Tested-by: Randy Dunlap <rdunlap@infradead.org> # build-tested
Signed-off-by: David S. Miller <davem@davemloft.net>