nfsd: Mark variable __maybe_unused to avoid W=1 build break

Clang is not happy about set but (in some cases) unused variable:

fs/nfsd/export.c:1027:17: error: variable 'inode' set but not used [-Werror,-Wunused-but-set-variable]

since it's used as a parameter to dprintk() which might be configured
a no-op. To avoid uglifying code with the specific ifdeffery just mark
the variable __maybe_unused.

The commit [1], which introduced this behaviour, is quite old and hence
the Fixes tag points to the first of the Git era.

Link: https://git.kernel.org/pub/scm/linux/kernel/git/history/history.git/commit/?id=0431923fb7a1 [1]
Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Cc: [e-mail]
Signed-off-by: Andy Shevchenko <[e-mail]>
Signed-off-by: Chuck Lever <[e-mail]>

fs/nfsd/export.c

···
 {
 	struct svc_export	*exp;
 	struct path		path;
-	struct inode		*inode;
+	struct inode		*inode __maybe_unused;
 	struct svc_fh		fh;
 	int			err;
 	struct nfsd_net		*nn = net_generic(net, nfsd_net_id);