mm: export add_swap_extent()

Btrfs currently does not support swap files because swap's use of bmap
does not work with copy-on-write and multiple devices.  See 35054394
("Btrfs: stop providing a bmap operation to avoid swapfile corruptions").

However, the swap code has a mechanism for the filesystem to manually add
swap extents using add_swap_extent() from the ->swap_activate() aop.
iomap has done this since 67482129 ("iomap: add a swapfile activation
function").  Btrfs will do the same in a later patch, so export
add_swap_extent().

Link: http://lkml.kernel.org/r/bb1208575e02829aae51b538709476964f97b1ea.1536704650.git.osandov@fb.comSigned-off-by: NOmar Sandoval <osandov@fb.com>
Reviewed-by: NAndrew Morton <akpm@linux-foundation.org>
Cc: David Sterba <dsterba@suse.cz>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

mm/swapfile.c

@@ -2354,6 +2354,7 @@ add_swap_extent(struct swap_info_struct *sis, unsigned long start_page,
 	list_add_tail(&new_se->list, &sis->first_swap_extent.list);
 	return 1;
 }
+EXPORT_SYMBOL_GPL(add_swap_extent);
 
 /*
  * A `swap extent' is a simple thing which maps a contiguous range of pages