buffer: Make bh_offset() work for compound pages

If the buffer pointed to by the buffer_head is part of a compound page,
bh_offset() assumes that b_page is the precise page that contains
the data.  A recent change to jbd2 inadvertently violated that assumption.

By using page_size(), we support both b_page being set to the head page
(as page_size() will return the size of the entire folio) and the precise
page (as it will return PAGE_SIZE for a tail page).

Fixes: 8147c4c4 ("jbd2: use a folio in jbd2_journal_write_metadata_buffer()")
Reported-by: Zorro Lang <zlang@kernel.org>
Tested-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>

include/linux/buffer_head.h

@@ -173,7 +173,10 @@ static __always_inline int buffer_uptodate(const struct buffer_head *bh)
 	return test_bit_acquire(BH_Uptodate, &bh->b_state);
 }
 
-#define bh_offset(bh)		((unsigned long)(bh)->b_data & ~PAGE_MASK)
+static inline unsigned long bh_offset(const struct buffer_head *bh)
+{
+	return (unsigned long)(bh)->b_data & (page_size(bh->b_page) - 1);
+}
 
 /* If we *know* page->private refers to buffer_heads */
 #define page_buffers(page)					\