sunrpc: suppress warnings for unused procfs functions

sunrpc: suppress warnings for unused procfs functions

There is a warning about unused variables when building with W=1 and no procfs:

net/sunrpc/cache.c:1660:30: error: 'cache_flush_proc_ops' defined but not used [-Werror=unused-const-variable=]
 1660 | static const struct proc_ops cache_flush_proc_ops = {
      |                              ^~~~~~~~~~~~~~~~~~~~
net/sunrpc/cache.c:1622:30: error: 'content_proc_ops' defined but not used [-Werror=unused-const-variable=]
 1622 | static const struct proc_ops content_proc_ops = {
      |                              ^~~~~~~~~~~~~~~~
net/sunrpc/cache.c:1598:30: error: 'cache_channel_proc_ops' defined but not used [-Werror=unused-const-variable=]
 1598 | static const struct proc_ops cache_channel_proc_ops = {
      |                              ^~~~~~~~~~~~~~~~~~~~~~

These are used inside of an #ifdef, so replacing that with an
IS_ENABLED() check lets the compiler see how they are used while
still dropping them during dead code elimination.

Fixes: dbf847ecb631 ("knfsd: allow cache_register to return error on failure")
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Acked-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Anna Schumaker <anna.schumaker@oracle.com>