btrfs: boilerplate: devlist and fsinfo

** This patch is not for integration but to debug/visualize the
btrfs device tree and fs_info. **

usage:
	cat /proc/fs/btrfs/devlist
	cat /proc/fs/btrfs/fsinfo

Signed-off-by: Anand Jain <anand.jain@oracle.com>

fs/btrfs/Makefile

@@ -11,7 +11,7 @@ btrfs-y += super.o ctree.o extent-tree.o print-tree.o root-tree.o dir-item.o \
 	   compression.o delayed-ref.o relocation.o delayed-inode.o scrub.o \
 	   reada.o backref.o ulist.o qgroup.o send.o dev-replace.o raid56.o \
 	   uuid-tree.o props.o free-space-tree.o tree-checker.o space-info.o \
-	   block-rsv.o delalloc-space.o block-group.o discard.o
+	   block-rsv.o delalloc-space.o block-group.o discard.o procfs.o
 
 btrfs-$(CONFIG_BTRFS_FS_POSIX_ACL) += acl.o
 btrfs-$(CONFIG_BTRFS_FS_CHECK_INTEGRITY) += check-integrity.o