filemap: Add a helper for filesystems implementing dropbehind

Add a helper to allow filesystems to attempt to free the 'dropbehind'
folio.

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Link: https://lore.kernel.org/all/5588a06f6d5a2cf6746828e2d36e7ada668b1739.1745381692.git.trond.myklebust@hammerspace.com/
Reviewed-by: Mike Snitzer <snitzer@kernel.org>
Signed-off-by: Anna Schumaker <anna.schumaker@oracle.com>

Author: Trond Myklebust

include/linux/pagemap.h

@@ -1221,6 +1221,7 @@ void folio_wait_writeback(struct folio *folio);
 int folio_wait_writeback_killable(struct folio *folio);
 void end_page_writeback(struct page *page);
 void folio_end_writeback(struct folio *folio);
+void folio_end_dropbehind(struct folio *folio);
 void folio_wait_stable(struct folio *folio);
 void __folio_mark_dirty(struct folio *folio, struct address_space *, int warn);
 void folio_account_cleaned(struct folio *folio, struct bdi_writeback *wb);

mm/filemap.c

@@ -1608,7 +1608,7 @@ static void filemap_end_dropbehind(struct folio *folio)
  * completes. Do that now. If we fail, it's likely because of a big folio -
  * just reset dropbehind for that case and latter completions should invalidate.
  */
-static void filemap_end_dropbehind_write(struct folio *folio)
+void folio_end_dropbehind(struct folio *folio)
 {
 	if (!folio_test_dropbehind(folio))
 		return;
@@ -1625,6 +1625,7 @@ static void filemap_end_dropbehind_write(struct folio *folio)
 		folio_unlock(folio);
 	}
 }
+EXPORT_SYMBOL_GPL(folio_end_dropbehind);
 
 /**
  * folio_end_writeback - End writeback against a folio.
@@ -1660,7 +1661,7 @@ void folio_end_writeback(struct folio *folio)
 	if (__folio_end_writeback(folio))
 		folio_wake_bit(folio, PG_writeback);
 
-	filemap_end_dropbehind_write(folio);
+	folio_end_dropbehind(folio);
 	acct_reclaim_writeback(folio);
 	folio_put(folio);
 }