fs/namespace.c: make to_mnt_ns() static

Make to_mnt_ns() static to address the following 'sparse' warning:

fs/namespace.c:1731:22: warning: symbol 'to_mnt_ns' was not declared. Should it be static?

Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Eric Biggers <[e-mail]>
Cc: Alexander Viro <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

fs/namespace.c

···
 	       dentry->d_fsdata == &mntns_operations;
 }
 
-struct mnt_namespace *to_mnt_ns(struct ns_common *ns)
+static struct mnt_namespace *to_mnt_ns(struct ns_common *ns)
 {
 	return container_of(ns, struct mnt_namespace, ns);
 }