mm: make mapping_get_entry available outside of filemap.c

mapping_get_entry is useful for page cache API users that need to know
about xa_value internals.  Rename it and make it available in pagemap.h.

Link: https://lkml.kernel.org/r/20230307143410.28031-3-hch@lst.deSigned-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Andreas Gruenbacher <agruenba@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/linux/pagemap.h

@@ -507,6 +507,7 @@ pgoff_t page_cache_prev_miss(struct address_space *mapping,
 #define FGP_ENTRY		0x00000080
 #define FGP_STABLE		0x00000100
 
+void *filemap_get_entry(struct address_space *mapping, pgoff_t index);
 struct folio *__filemap_get_folio(struct address_space *mapping, pgoff_t index,
 		int fgp_flags, gfp_t gfp);
 struct page *pagecache_get_page(struct address_space *mapping, pgoff_t index,

mm/filemap.c

@@ -1836,7 +1836,7 @@ EXPORT_SYMBOL(page_cache_prev_miss);
  */
 
 /*
- * mapping_get_entry - Get a page cache entry.
+ * filemap_get_entry - Get a page cache entry.
  * @mapping: the address_space to search
  * @index: The page cache index.
  *
@@ -1847,7 +1847,7 @@ EXPORT_SYMBOL(page_cache_prev_miss);
  *
  * Return: The folio, swap or shadow entry, %NULL if nothing is found.
  */
-static void *mapping_get_entry(struct address_space *mapping, pgoff_t index)
+void *filemap_get_entry(struct address_space *mapping, pgoff_t index)
 {
 	XA_STATE(xas, &mapping->i_pages, index);
 	struct folio *folio;
@@ -1917,7 +1917,7 @@ struct folio *__filemap_get_folio(struct address_space *mapping, pgoff_t index,
 	struct folio *folio;
 
 repeat:
-	folio = mapping_get_entry(mapping, index);
+	folio = filemap_get_entry(mapping, index);
 	if (xa_is_value(folio)) {
 		if (fgp_flags & FGP_ENTRY)
 			return folio;