Linux 5.16 compat: Added add_disk check for return

add_disk went from void to must-check int return.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Coleman Kane <ckane@colemankane.org>
Signed-off-by: Rich Ercolani <rincebrain@gmail.com>
Closes openzfs#12975

config/kernel-add-disk.m4

@@ -0,0 +1,27 @@
+dnl #
+dnl # 5.16 API change
+dnl # add_disk grew a must-check return code
+dnl #
+AC_DEFUN([ZFS_AC_KERNEL_SRC_ADD_DISK], [
+
+	ZFS_LINUX_TEST_SRC([add_disk_ret], [
+		#include <linux/genhd.h>
+	], [
+		struct gendisk *disk = NULL;
+		int err = add_disk(disk);
+		err = err;
+	])
+
+])
+
+AC_DEFUN([ZFS_AC_KERNEL_ADD_DISK], [
+	AC_MSG_CHECKING([whether add_disk() returns int])
+	ZFS_LINUX_TEST_RESULT([add_disk_ret],
+	[
+		AC_MSG_RESULT(yes)
+		AC_DEFINE(HAVE_ADD_DISK_RET, 1,
+		    [add_disk() returns int])
+	], [
+		AC_MSG_RESULT(no)
+	])
+])

config/kernel.m4

@@ -135,6 +135,7 @@ AC_DEFUN([ZFS_AC_KERNEL_TEST_SRC], [
 	ZFS_AC_KERNEL_SRC_VFS_SET_PAGE_DIRTY_NOBUFFERS
 	ZFS_AC_KERNEL_SRC_STANDALONE_LINUX_STDARG
 	ZFS_AC_KERNEL_SRC_PAGEMAP_FOLIO_WAIT_BIT
+	ZFS_AC_KERNEL_SRC_ADD_DISK
 
 	AC_MSG_CHECKING([for available kernel interfaces])
 	ZFS_LINUX_TEST_COMPILE_ALL([kabi])
@@ -243,6 +244,7 @@ AC_DEFUN([ZFS_AC_KERNEL_TEST_RESULT], [
 	ZFS_AC_KERNEL_VFS_SET_PAGE_DIRTY_NOBUFFERS
 	ZFS_AC_KERNEL_STANDALONE_LINUX_STDARG
 	ZFS_AC_KERNEL_PAGEMAP_FOLIO_WAIT_BIT
+	ZFS_AC_KERNEL_ADD_DISK
 ])
 
 dnl #

module/os/linux/zfs/zvol_os.c

@@ -1110,7 +1110,11 @@ zvol_os_create_minor(const char *name)
 		rw_enter(&zvol_state_lock, RW_WRITER);
 		zvol_insert(zv);
 		rw_exit(&zvol_state_lock);
+#ifdef HAVE_ADD_DISK_RET
+		error = add_disk(zv->zv_zso->zvo_disk);
+#else
 		add_disk(zv->zv_zso->zvo_disk);
+#endif
 	} else {
 		ida_simple_remove(&zvol_ida, idx);
 	}