list: remove redundant 'extern' for function prototypes

The 'extern' keyword is redundant for function prototypes.  list.h never
used them and new code in general is better without them.

Link: https://lkml.kernel.org/r/20250502121742.3997529-1-andriy.shevchenko@linux.intel.com
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: andy-shev

include/linux/list.h

@@ -50,9 +50,9 @@ static inline void INIT_LIST_HEAD(struct list_head *list)
  * Performs the full set of list corruption checks before __list_add().
  * On list corruption reports a warning, and returns false.
  */
-extern bool __list_valid_slowpath __list_add_valid_or_report(struct list_head *new,
-							     struct list_head *prev,
-							     struct list_head *next);
+bool __list_valid_slowpath __list_add_valid_or_report(struct list_head *new,
+						      struct list_head *prev,
+						      struct list_head *next);
 
 /*
  * Performs list corruption checks before __list_add(). Returns false if a
@@ -93,7 +93,7 @@ static __always_inline bool __list_add_valid(struct list_head *new,
  * Performs the full set of list corruption checks before __list_del_entry().
  * On list corruption reports a warning, and returns false.
  */
-extern bool __list_valid_slowpath __list_del_entry_valid_or_report(struct list_head *entry);
+bool __list_valid_slowpath __list_del_entry_valid_or_report(struct list_head *entry);
 
 /*
  * Performs list corruption checks before __list_del_entry(). Returns false if a