Linux 4.12 compat: super_setup_bdi_name()

All filesystems were converted to dynamically allocated BDIs.  The
destruction of backing_dev_info structures is handled as part of
super block destruction.  Refactor the code to abstract away the
details of creating and destroying a BDI.

Reviewed-by: Chunwei Chen <david.chen@osnexus.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #6089

include/sys/zfs_vfsops.h

@@ -75,7 +75,6 @@ typedef struct zfs_mnt {
 struct zfsvfs {
 	vfs_t		*z_vfs;		/* generic fs struct */
 	struct super_block *z_sb;	/* generic super_block */
-	struct backing_dev_info z_bdi;	/* generic backing dev info */
 	struct zfsvfs	*z_parent;	/* parent fs */
 	objset_t	*z_os;		/* objset reference */
 	uint64_t	z_flags;	/* super_block flags */