fscrypt: convert bh_get_inode_and_lblk_num to use a folio

Patch series "Remove page_mapping()".

There are only a few users left.  Convert them all to either call
folio_mapping() or just use folio->mapping directly.  


This patch (of 6):

Remove uses of page->index, page_mapping() and b_page.  Saves a call
to compound_head().

Link: https://lkml.kernel.org/r/20240423225552.4113447-1-willy@infradead.org
Link: https://lkml.kernel.org/r/20240423225552.4113447-2-willy@infradead.orgSigned-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Cc: Sidhartha Kumar <sidhartha.kumar@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

fs/crypto/inline_crypt.c

@@ -284,7 +284,7 @@ static bool bh_get_inode_and_lblk_num(const struct buffer_head *bh,
 				      const struct inode **inode_ret,
 				      u64 *lblk_num_ret)
 {
-	struct page *page = bh->b_page;
+	struct folio *folio = bh->b_folio;
 	const struct address_space *mapping;
 	const struct inode *inode;
 
@@ -292,13 +292,13 @@ static bool bh_get_inode_and_lblk_num(const struct buffer_head *bh,
 	 * The ext4 journal (jbd2) can submit a buffer_head it directly created
 	 * for a non-pagecache page.  fscrypt doesn't care about these.
 	 */
-	mapping = page_mapping(page);
+	mapping = folio_mapping(folio);
 	if (!mapping)
 		return false;
 	inode = mapping->host;
 
 	*inode_ret = inode;
-	*lblk_num_ret = ((u64)page->index << (PAGE_SHIFT - inode->i_blkbits)) +
+	*lblk_num_ret = ((u64)folio->index << (PAGE_SHIFT - inode->i_blkbits)) +
 			(bh_offset(bh) >> inode->i_blkbits);
 	return true;
 }