Allow mounting datasets more than once

Currently mounting an already mounted zfs dataset results in an
error, whereas it is typically allowed with other filesystems.
This causes some bad interactions with mount namespaces. Take
this sequence for example:

- Create a dataset
- Create a snapshot of the dataset
- Create a clone of the snapshot
- Create a new mount namespace
- Rename the original dataset

The rename results in unmounting and remounting the clone in the
original mount namespace, however the remount fails because the
dataset is still mounted in the new mount namespace. (Note that
this means the mount in the new mount namespace is never being
unmounted, so perhaps the unmount/remount of the clone isn't
actually necessary.)

The problem here is a result of the way mounting is implemented
in the kernel module. Since it is not mounting block devices it
uses mount_nodev() instead of the usual mount_bdev(). However,
mount_nodev() is written for filesystems for which each mount is
a new instance (i.e. a new super block), and zfs should be able
to detect when a mount request can be satisfied using an existing
super block.

Change zpl_mount() to call sget() directly with it's own test
callback. Passing the objset_t object as the fs data allows
checking if a superblock already exists for the dataset, and in
that case we just need to return a new reference for the sb's
root dentry.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tom Caputi <tcaputi@datto.com>
Signed-off-by: Alek Pinchuk <apinchuk@datto.com>
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
Closes #5796
Closes #7207

include/linux/vfs_compat.h

@@ -182,6 +182,30 @@ zpl_bdi_destroy(struct super_block *sb)
 }
 #endif
 
+/*
+ * 4.14 adds SB_* flag definitions, define them to MS_* equivalents
+ * if not set.
+ */
+#ifndef	SB_RDONLY
+#define	SB_RDONLY	MS_RDONLY
+#endif
+
+#ifndef	SB_SILENT
+#define	SB_SILENT	MS_SILENT
+#endif
+
+#ifndef	SB_ACTIVE
+#define	SB_ACTIVE	MS_ACTIVE
+#endif
+
+#ifndef	SB_POSIXACL
+#define	SB_POSIXACL	MS_POSIXACL
+#endif
+
+#ifndef	SB_MANDLOCK
+#define	SB_MANDLOCK	MS_MANDLOCK
+#endif
+
 /*
  * 2.6.38 API change,
  * LOOKUP_RCU flag introduced to distinguish rcu-walk from ref-walk cases.