Linux 5.16: The blk-cgroup.h header is where struct blkcg_gq is defined

The definition of struct blkcg_gq was moved into blk-cgroup.h, which is
a header that's been in Linux since 2015. This is used by
vdev_blkg_tryget() in module/os/linux/zfs/vdev_disk.c. Since the kernel
for CentOS 7 and similar-generation releases doesn't have this header,
its inclusion is guarded by a configure test.

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Coleman Kane <ckane@colemankane.org>
Closes #12819