Linux 6.8 compat: handle mnt_idmap user_namespace change

struct mnt_idmap no longer has a struct user_namespace within it. Work
around this by creating a temporary with the copy of the map we need
taken from the idmap.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Co-authored-by: Youzhong Yang <yyang@mathworks.com>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Sponsored-by: https://despairlabs.com/sponsor/
Closes #15805

config/kernel-idmap_mnt_api.m4

@@ -23,3 +23,28 @@ AC_DEFUN([ZFS_AC_KERNEL_IDMAP_MNT_API], [
         ])
 ])
 
+dnl #
+dnl # 6.8 decouples mnt_idmap from user_namespace. This is all internal
+dnl # to mnt_idmap so we can't detect it directly, but we detect a related
+dnl # change as use that as a signal.
+dnl #
+AC_DEFUN([ZFS_AC_KERNEL_SRC_IDMAP_NO_USERNS], [
+	ZFS_LINUX_TEST_SRC([idmap_no_userns], [
+		#include <linux/uidgid.h>
+	], [
+		struct uid_gid_map *map = NULL;
+		map_id_down(map, 0);
+	])
+])
+
+
+AC_DEFUN([ZFS_AC_KERNEL_IDMAP_NO_USERNS], [
+	AC_MSG_CHECKING([whether idmapped mounts have a user namespace])
+	ZFS_LINUX_TEST_RESULT([idmap_no_userns], [
+		AC_MSG_RESULT([yes])
+		AC_DEFINE(HAVE_IDMAP_NO_USERNS, 1,
+			[mnt_idmap does not have user_namespace])
+	], [
+		AC_MSG_RESULT([no])
+	])
+])

config/kernel.m4

@@ -158,6 +158,7 @@ AC_DEFUN([ZFS_AC_KERNEL_TEST_SRC], [
 	ZFS_AC_KERNEL_SRC___COPY_FROM_USER_INATOMIC
 	ZFS_AC_KERNEL_SRC_USER_NS_COMMON_INUM
 	ZFS_AC_KERNEL_SRC_IDMAP_MNT_API
+	ZFS_AC_KERNEL_SRC_IDMAP_NO_USERNS
 	ZFS_AC_KERNEL_SRC_IATTR_VFSID
 	ZFS_AC_KERNEL_SRC_FILEMAP
 	ZFS_AC_KERNEL_SRC_WRITEPAGE_T
@@ -305,6 +306,7 @@ AC_DEFUN([ZFS_AC_KERNEL_TEST_RESULT], [
 	ZFS_AC_KERNEL___COPY_FROM_USER_INATOMIC
 	ZFS_AC_KERNEL_USER_NS_COMMON_INUM
 	ZFS_AC_KERNEL_IDMAP_MNT_API
+	ZFS_AC_KERNEL_IDMAP_NO_USERNS
 	ZFS_AC_KERNEL_IATTR_VFSID
 	ZFS_AC_KERNEL_FILEMAP
 	ZFS_AC_KERNEL_WRITEPAGE_T