nfsd: eliminate NFSD_DEBUG

Commit f895b252 ("sunrpc: eliminate RPC_DEBUG") introduced
use of IS_ENABLED() in a uapi header which leads to a build
failure for userspace apps trying to use <linux/nfsd/debug.h>:

   linux/nfsd/debug.h:18:15: error: missing binary operator before token "("
  #if IS_ENABLED(CONFIG_SUNRPC_DEBUG)
                ^

Since this was only used to define NFSD_DEBUG if CONFIG_SUNRPC_DEBUG
is enabled, replace instances of NFSD_DEBUG with CONFIG_SUNRPC_DEBUG.

Cc: stable@vger.kernel.org
Fixes: f895b252

 "sunrpc: eliminate RPC_DEBUG"
Signed-off-by: Mark Salter <msalter@redhat.com>
Reviewed-by: Jeff Layton <jlayton@primarydata.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>