Linux: Fix zfs_prune panics v2

It turns out that approach taken in the original version of the patch
was wrong. So now, we're taking approach in-line with how kernel
actually does it - when sb is being torn down, access to it
is serialized via sb->s_umount rwsem, only when that lock is taken
is it okay to work with s_flags - and the other mistake I was doing
was trying to make SB_ACTIVE work, but apparently the kernel checks
the negative variant - not SB_DYING and not SB_BORN.

Kernels pre-6.6 don't have SB_DYING, but check if sb is hashed
instead.

Signed-off-by: Pavel Snajdr <snajpa@snajpa.net>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes: #17121
(cherry picked from commit a0e62718cf)

config/kernel-sb-dying.m4

@@ -0,0 +1,19 @@
+dnl #
+dnl # SB_DYING exists since Linux 6.6
+dnl #
+AC_DEFUN([ZFS_AC_KERNEL_SRC_SB_DYING], [
+	ZFS_LINUX_TEST_SRC([sb_dying], [
+		#include <linux/fs.h>
+	],[
+		(void) SB_DYING;
+	])
+])
+
+AC_DEFUN([ZFS_AC_KERNEL_SB_DYING], [
+	AC_MSG_CHECKING([whether SB_DYING is defined])
+	ZFS_LINUX_TEST_RESULT([sb_dying], [
+		AC_MSG_RESULT(yes)
+	],[
+		AC_MSG_RESULT(no)
+	])
+])

config/kernel.m4

@@ -73,6 +73,7 @@ AC_DEFUN([ZFS_AC_KERNEL_TEST_SRC], [
 	ZFS_AC_KERNEL_SRC_TRUNCATE_SETSIZE
 	ZFS_AC_KERNEL_SRC_SECURITY_INODE
 	ZFS_AC_KERNEL_SRC_FST_MOUNT
+	ZFS_AC_KERNEL_SRC_SB_DYING
 	ZFS_AC_KERNEL_SRC_SET_NLINK
 	ZFS_AC_KERNEL_SRC_SGET
 	ZFS_AC_KERNEL_SRC_VFS_FILEMAP_DIRTY_FOLIO
@@ -183,6 +184,7 @@ AC_DEFUN([ZFS_AC_KERNEL_TEST_RESULT], [
 	ZFS_AC_KERNEL_TRUNCATE_SETSIZE
 	ZFS_AC_KERNEL_SECURITY_INODE
 	ZFS_AC_KERNEL_FST_MOUNT
+	ZFS_AC_KERNEL_SB_DYING
 	ZFS_AC_KERNEL_SET_NLINK
 	ZFS_AC_KERNEL_SGET
 	ZFS_AC_KERNEL_VFS_FILEMAP_DIRTY_FOLIO