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

module/os/linux/zfs/vdev_disk.c

@@ -37,6 +37,9 @@
 #include <linux/blkpg.h>
 #include <linux/msdos_fs.h>
 #include <linux/vfs_compat.h>
+#ifdef HAVE_LINUX_BLK_CGROUP_HEADER
+#include <linux/blk-cgroup.h>
+#endif
 
 typedef struct vdev_disk {
 	struct block_device		*vd_bdev;