[PATCH for-mm-unstable] mm/swap.h: Define swap_cache_get_folio() for !CONFIG_SWAP

[SeongJae Park <sj@kernel.org>]

Commit 4d3cb611e6df ("swap: add swap_cache_get_folio()") in mm-unstable
tree defines 'swap_cache_get_folio()' for CONFIG_SWAP only, and as a
result build fails on !CONFIG_SWAP as below, after following commits
using the function under !CONFIG_SWAP is applied.

    mm/shmem.c: In function ‘shmem_swapin_folio’:
    mm/shmem.c:1738:10: error: implicit declaration of function ‘swap_cache_get_folio’; did you mean ‘read_cache_folio’? [-Werror=implicit-function-declaration]
     1738 |  folio = swap_cache_get_folio(swap, NULL, 0);
          |          ^~~~~~~~~~~~~~~~~~~~
          |          read_cache_folio
    mm/shmem.c:1738:8: warning: assignment to ‘struct folio *’ from ‘int’ makes pointer from integer without a cast [-Wint-conversion]
     1738 |  folio = swap_cache_get_folio(swap, NULL, 0);
          |        ^
    cc1: some warnings being treated as errors

This commit fixes the issue by defining the function for !CONFIG_SWAP
case.

Fixes:4d3cb611e6df ("swap: add swap_cache_get_folio()") in mm-unstable (1e6b789996e7)
Signed-off-by: SeongJae Park <sj@kernel.org>
---

Note: This patch does not cleanly applicable on top of the mm-unstable
but the fixing commit.

 mm/swap.h | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/mm/swap.h b/mm/swap.h
index 2d6fe6c44ad9..ccd8d9a9ad36 100644
--- a/mm/swap.h
+++ b/mm/swap.h
@@ -101,6 +101,12 @@ static inline int swap_writepage(struct page *p, struct writeback_control *wbc)
 	return 0;
 }
 
+static inline struct folio *swap_cache_get_folio(swp_entry_t entry,
+		struct vm_area_struct *vma, unsigned long addr)
+{
+	return NULL;
+}
+
 static inline struct page *lookup_swap_cache(swp_entry_t swp,
 					     struct vm_area_struct *vma,
 					     unsigned long addr)
-- 
2.25.1