cgroup, blkcg: Prepare some symbols for module and !CONFIG_CGROUP usages

commit 9b0eb69b upstream.

btrfs is going to use css_put() and wbc helpers to improve cgroup
writeback support.  Add dummy css_get() definition and export wbc
helpers to prepare for module and !CONFIG_CGROUP builds.

[only backport the export of __inode_attach_wb for stable kernels - gregkh]

Reported-by: kbuild test robot <lkp@intel.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
[PG: borrow Greg's stable fix from v4.19 for v5.2.x here.]
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>