fs: Constify path_is_under()'s arguments

The function path_is_under() doesn't modify the paths pointed by its
arguments but only browse them. Constifying this pointers make a cleaner
interface to be used by (future) code which may only have access to
const struct path pointers (e.g. LSM hooks).

Signed-off-by: Mickaël Salaün <mic@digikod.net>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/namespace.c

@@ -2997,7 +2997,7 @@ bool is_path_reachable(struct mount *mnt, struct dentry *dentry,
 	return &mnt->mnt == root->mnt && is_subdir(dentry, root->dentry);
 }
 
-bool path_is_under(struct path *path1, struct path *path2)
+bool path_is_under(const struct path *path1, const struct path *path2)
 {
 	bool res;
 	read_seqlock_excl(&mount_lock);