fs/mpage: avoid negative shift for large blocksize

For large blocksizes the number of block bits is larger than PAGE_SHIFT,
so calculate the sector number from the byte offset instead. This is
required to enable large folios with buffer-heads.

Reviewed-by: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
Signed-off-by: Hannes Reinecke <hare@kernel.org>
Link: https://lore.kernel.org/r/20250221223823.1680616-4-mcgrof@kernel.org


Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Christian Brauner <brauner@kernel.org>