fs/buffer: simplify block_read_full_folio() with bh_offset()

fs/buffer: simplify block_read_full_folio() with bh_offset()

When we read over all buffers in a folio we currently use the
buffer index on the folio and blocksize to get the offset. Simplify
this with bh_offset(). This simplifies the loop while making no
functional changes.

Suggested-by: Matthew Wilcox <willy@infradead.org>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
Link: https://lore.kernel.org/r/20250221223823.1680616-2-mcgrof@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>