fs_context flavour for submounts

This is an eventual replacement for vfs_submount() uses.  Unlike the
"mount" and "remount" cases, the users of that thing are not in VFS -
they are buried in various ->d_automount() instances and rather than
converting them all at once we introduce the (thankfully small and
simple) infrastructure here and deal with the prospective users in
afs, nfs, etc. parts of the series.

Here we just introduce a new constructor (fs_context_for_submount())
along with the corresponding enum constant to be put into fc->purpose
for those.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/fs_context.c

@@ -69,6 +69,9 @@ static struct fs_context *alloc_fs_context(struct file_system_type *fs_type,
 	case FS_CONTEXT_FOR_MOUNT:
 		fc->user_ns = get_user_ns(fc->cred->user_ns);
 		break;
+	case FS_CONTEXT_FOR_SUBMOUNT:
+		fc->user_ns = get_user_ns(reference->d_sb->s_user_ns);
+		break;
 	case FS_CONTEXT_FOR_RECONFIGURE:
 		/* We don't pin any namespaces as the superblock's
 		 * subscriptions cannot be changed at this point.
@@ -106,6 +109,13 @@ struct fs_context *fs_context_for_reconfigure(struct dentry *dentry,
 }
 EXPORT_SYMBOL(fs_context_for_reconfigure);
 
+struct fs_context *fs_context_for_submount(struct file_system_type *type,
+					   struct dentry *reference)
+{
+	return alloc_fs_context(type, reference, 0, 0, FS_CONTEXT_FOR_SUBMOUNT);
+}
+EXPORT_SYMBOL(fs_context_for_submount);
+
 void fc_drop_locked(struct fs_context *fc)
 {
 	struct super_block *sb = fc->root->d_sb;

include/linux/fs_context.h

@@ -25,6 +25,7 @@ struct user_namespace;
 
 enum fs_context_purpose {
 	FS_CONTEXT_FOR_MOUNT,		/* New superblock for explicit mount */
+	FS_CONTEXT_FOR_SUBMOUNT,	/* New superblock for automatic submount */
 	FS_CONTEXT_FOR_RECONFIGURE,	/* Superblock reconfiguration (remount) */
 };
 
@@ -61,6 +62,8 @@ extern struct fs_context *fs_context_for_mount(struct file_system_type *fs_type,
 extern struct fs_context *fs_context_for_reconfigure(struct dentry *dentry,
 						unsigned int sb_flags,
 						unsigned int sb_flags_mask);
+extern struct fs_context *fs_context_for_submount(struct file_system_type *fs_type,
+						struct dentry *reference);
 
 extern int vfs_get_tree(struct fs_context *fc);
 extern void put_fs_context(struct fs_context *fc);