fs: namespace: Avoid -Wflex-array-member-not-at-end warning

-Wflex-array-member-not-at-end was introduced in GCC-14, and we are
getting ready to enable it, globally.

Move the conflicting declaration to the end of the structure. Notice
that `struct statmount` is a flexible structure --a structure that
contains a flexible-array member.

Fix the following warning:

fs/namespace.c:5329:26: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]

Signed-off-by: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/Z-SZKNdCiAkVJvqm@kspp
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: GustavoARSilva

fs/namespace.c

@@ -5326,8 +5326,10 @@ struct kstatmount {
 	struct mnt_idmap *idmap;
 	u64 mask;
 	struct path root;
-	struct statmount sm;
 	struct seq_file seq;
+
+	/* Must be last --ends in a flexible-array member. */
+	struct statmount sm;
 };
 
 static u64 mnt_to_attr_flags(struct vfsmount *mnt)