zpl_sync_fs: work around kernels that ignore sync_fs errors

If the kernel will honour our error returns, use them. If not, fool it
by setting a writeback error on the superblock, if available.

Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.
Reviewed-by: Paul Dagnelie <paul.dagnelie@klarasystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
Closes #17420

config/kernel-sb-wb-err.m4

@@ -0,0 +1,27 @@
+# dnl
+# dnl 5.8 (735e4ae5ba28) introduced a superblock scoped errseq_t to use to
+# dnl record writeback errors for syncfs() to return. Up until 5.17, when
+# dnl sync_fs errors were returned directly, this is the only way for us to
+# dnl report an error from syncfs().
+# dnl
+AC_DEFUN([ZFS_AC_KERNEL_SRC_SUPER_BLOCK_S_WB_ERR], [
+	ZFS_LINUX_TEST_SRC([super_block_s_wb_err], [
+		#include <linux/fs.h>
+
+		static const struct super_block
+		    sb __attribute__ ((unused)) = {
+			.s_wb_err = 0,
+		};
+	],[])
+])
+
+AC_DEFUN([ZFS_AC_KERNEL_SUPER_BLOCK_S_WB_ERR], [
+	AC_MSG_CHECKING([whether super_block has s_wb_err])
+	ZFS_LINUX_TEST_RESULT([super_block_s_wb_err], [
+		AC_MSG_RESULT(yes)
+		AC_DEFINE(HAVE_SUPER_BLOCK_S_WB_ERR, 1,
+			[have super_block s_wb_err])
+	],[
+		AC_MSG_RESULT(no)
+	])
+])

config/kernel.m4

@@ -131,6 +131,7 @@ AC_DEFUN([ZFS_AC_KERNEL_TEST_SRC], [
 	ZFS_AC_KERNEL_SRC_FILE
 	ZFS_AC_KERNEL_SRC_PIN_USER_PAGES
 	ZFS_AC_KERNEL_SRC_TIMER
+	ZFS_AC_KERNEL_SRC_SUPER_BLOCK_S_WB_ERR
 	case "$host_cpu" in
 		powerpc*)
 			ZFS_AC_KERNEL_SRC_CPU_HAS_FEATURE
@@ -246,6 +247,7 @@ AC_DEFUN([ZFS_AC_KERNEL_TEST_RESULT], [
 	ZFS_AC_KERNEL_FILE
 	ZFS_AC_KERNEL_PIN_USER_PAGES
 	ZFS_AC_KERNEL_TIMER
+	ZFS_AC_KERNEL_SUPER_BLOCK_S_WB_ERR
 	case "$host_cpu" in
 		powerpc*)
 			ZFS_AC_KERNEL_CPU_HAS_FEATURE