switch the IO-triggering parts of umount to fs_pin

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/fs/mount.h b/fs/mount.h
index 0ad6f76..6a61c2b 100644
--- a/fs/mount.h
+++ b/fs/mount.h
@@ -2,6 +2,7 @@
 #include <linux/seq_file.h>
 #include <linux/poll.h>
 #include <linux/ns_common.h>
+#include <linux/fs_pin.h>
 
 struct mnt_namespace {
 	atomic_t		count;
@@ -62,7 +63,8 @@
 	int mnt_group_id;		/* peer group identifier */
 	int mnt_expiry_mark;		/* true if marked for expiry */
 	struct hlist_head mnt_pins;
-	struct path mnt_ex_mountpoint;
+	struct fs_pin mnt_umount;
+	struct dentry *mnt_ex_mountpoint;
 };
 
 #define MNT_NS_INTERNAL ERR_PTR(-EINVAL) /* distinct from any mnt_namespace */