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>