zfs_context: move vn_dumpdir to libzpool

Sponsored-by: https://despairlabs.com/sponsor/
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Closes #17861

include/libzpool.h

@@ -35,6 +35,8 @@
 extern "C" {
 #endif
 
+extern char *vn_dumpdir;
+
 _LIBZPOOL_H void kernel_init(int mode);
 _LIBZPOOL_H void kernel_fini(void);
 

include/sys/zfs_context.h

@@ -187,8 +187,6 @@ extern "C" {
 typedef off_t loff_t;
 #endif
 
-extern char *vn_dumpdir;
-
 /*
  * Random stuff
  */