vfs: Create fs_context-aware mount_bdev() replacement

Create a function, get_tree_bdev(), that is fs_context-aware and a
->get_tree() counterpart of mount_bdev().

It caches the block device pointer in the fs_context struct so that this
information can be passed into sget_fc()'s test and set functions.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Jens Axboe <axboe@kernel.dk>
cc: linux-block@vger.kernel.org
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

include/linux/fs_context.h

@@ -88,6 +88,7 @@ struct fs_context {
 	struct mutex		uapi_mutex;	/* Userspace access mutex */
 	struct file_system_type	*fs_type;
 	void			*fs_private;	/* The filesystem's context */
+	void			*sget_key;
 	struct dentry		*root;		/* The root and superblock */
 	struct user_namespace	*user_ns;	/* The user namespace for this mount */
 	struct net		*net_ns;	/* The network namespace for this mount */
@@ -159,6 +160,10 @@ extern int get_tree_keyed(struct fs_context *fc,
 					   struct fs_context *fc),
 			 void *key);
 
+extern int get_tree_bdev(struct fs_context *fc,
+			       int (*fill_super)(struct super_block *sb,
+						 struct fs_context *fc));
+
 extern const struct file_operations fscontext_fops;
 
 /*