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 openzfs#12819

config/kernel-bio.m4

@@ -449,6 +449,31 @@ AC_DEFUN([ZFS_AC_KERNEL_BDEV_SUBMIT_BIO_RETURNS_VOID], [
 	])
 ])
 
+dnl #
+dnl # Linux 5.16 API
+dnl #
+dnl # The Linux 5.16 API moved struct blkcg_gq into linux/blk-cgroup.h, which
+dnl # has been around since 2015. This test looks for the presence of that
+dnl # header, so that it can be conditionally included where it exists, but
+dnl # still be backward compatible with kernels that pre-date its introduction.
+dnl #
+AC_DEFUN([ZFS_AC_KERNEL_SRC_BLK_CGROUP_HEADER], [
+	ZFS_LINUX_TEST_SRC([blk_cgroup_header], [
+		#include <linux/blk-cgroup.h>
+	], [])
+])
+
+AC_DEFUN([ZFS_AC_KERNEL_BLK_CGROUP_HEADER], [
+	AC_MSG_CHECKING([for existence of linux/blk-cgroup.h])
+	ZFS_LINUX_TEST_RESULT([blk_cgroup_header],[
+		AC_MSG_RESULT(yes)
+		AC_DEFINE(HAVE_LINUX_BLK_CGROUP_HEADER, 1,
+			[linux/blk-cgroup.h exists])
+	],[
+		AC_MSG_RESULT(no)
+	])
+])
+
 AC_DEFUN([ZFS_AC_KERNEL_SRC_BIO], [
 	ZFS_AC_KERNEL_SRC_REQ
 	ZFS_AC_KERNEL_SRC_BIO_OPS
@@ -462,6 +487,7 @@ AC_DEFUN([ZFS_AC_KERNEL_SRC_BIO], [
 	ZFS_AC_KERNEL_SRC_BIO_BDEV_DISK
 	ZFS_AC_KERNEL_SRC_BDEV_SUBMIT_BIO_RETURNS_VOID
 	ZFS_AC_KERNEL_SRC_BIO_SET_DEV_MACRO
+	ZFS_AC_KERNEL_SRC_BLK_CGROUP_HEADER
 ])
 
 AC_DEFUN([ZFS_AC_KERNEL_BIO], [
@@ -485,4 +511,5 @@ AC_DEFUN([ZFS_AC_KERNEL_BIO], [
 	ZFS_AC_KERNEL_BLKG_TRYGET
 	ZFS_AC_KERNEL_BIO_BDEV_DISK
 	ZFS_AC_KERNEL_BDEV_SUBMIT_BIO_RETURNS_VOID
+	ZFS_AC_KERNEL_BLK_CGROUP_HEADER
 ])

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;