Skip __zfs_dbgmsg in a DPC (#104)

* Skip dprintf in a DPC

Add a check to dprintf that will avoid trying to write to the debug log if we are in a IRQL of DISPATCH_LEVEL or higher.
Otherwise we will get a BSOD when __zfs_dbgmsg tries to grab a mutex.
#103

* dprintf avoid bsod while still providing information

* Revert: dprintf avoid bsod while still providing information

module/os/windows/zfs/zfs_debug.c

@@ -203,6 +203,13 @@ __dprintf(boolean_t dprint, const char *file, const char *func,
 	char *prefix = (dprint) ? "dprintf: " : "";
 	const char *newfile;
 
+	/*
+	 * Skip everything if we can't write to the debug log due to
+	 * being in a DPC.
+	 */
+	if (KeGetCurrentIrql() >= DISPATCH_LEVEL)
+		return;
+
 	/*
 	 * Get rid of annoying prefix to filename.
 	 */