mm: mark swap_lock and swap_active_head static

swap_lock and swap_active_head are only used in swapfile.c, so mark them
static.

Link: https://lkml.kernel.org/r/20211224062246.1258487-12-hch@lst.de
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Juergen Gross <jgross@suse.com>
Cc: Dan Streetman <ddstreet@ieee.org>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: Konrad Rzeszutek Wilk <Konrad.wilk@oracle.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Seth Jennings <sjenning@redhat.com>
Cc: Vitaly Wool <vitaly.wool@konsulko.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Author: Christoph Hellwig

include/linux/swapfile.h

@@ -6,8 +6,6 @@
  * these were static in swapfile.c but frontswap.c needs them and we don't
  * want to expose them to the dozens of source files that include swap.h
  */
-extern spinlock_t swap_lock;
-extern struct plist_head swap_active_head;
 extern struct swap_info_struct *swap_info[];
 extern unsigned long generic_max_swapfile_size(void);
 extern unsigned long max_swapfile_size(void);

mm/swapfile.c

@@ -49,7 +49,7 @@ static bool swap_count_continued(struct swap_info_struct *, pgoff_t,
 				 unsigned char);
 static void free_swap_count_continuations(struct swap_info_struct *);
 
-DEFINE_SPINLOCK(swap_lock);
+static DEFINE_SPINLOCK(swap_lock);
 static unsigned int nr_swapfiles;
 atomic_long_t nr_swap_pages;
 /*
@@ -71,7 +71,7 @@ static const char Unused_offset[] = "Unused swap offset entry ";
  * all active swap_info_structs
  * protected with swap_lock, and ordered by priority.
  */
-PLIST_HEAD(swap_active_head);
+static PLIST_HEAD(swap_active_head);
 
 /*
  * all available (active, not full) swap_info_structs