log: fixup log_head after relocating global data

When `gd` is relocated during `spl_relocate_stack_gd()` the
doubly-linked circular list in the `log_head` member is broken.

The last element of the list should point back to the initial
`list_head`, but as the initial `list_head` is moved the pointer becomes
stale. As a result the loop in `log_dispatch` would never finish.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>

include/log.h

@@ -688,4 +688,16 @@ static inline int log_get_default_format(void)
 	       (IS_ENABLED(CONFIG_LOGF_FUNC) ? BIT(LOGF_FUNC) : 0);
 }
 
+struct global_data;
+/**
+ * log_fixup_for_gd_move() - Handle global_data moving to a new place
+ *
+ * @new_gd: Pointer to the new global data
+ *
+ * The log_head list is part of global_data. Due to the way lists work, moving
+ * the list will cause it to become invalid. This function fixes that up so
+ * that the log_head list will work correctly.
+ */
+void log_fixup_for_gd_move(struct global_data *new_gd);
+
 #endif