buffer: cast block to loff_t before shifting it

buffer: cast block to loff_t before shifting it

While sector_t is always defined as a u64 today, that hasn't always been
the case and it might not always be the same size as loff_t in the future.

Link: https://lkml.kernel.org/r/20231109210608.2252323-5-willy@infradead.org
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Hannes Reinecke <hare@suse.de>
Cc: Luis Chamberlain <mcgrof@kernel.org>
Cc: Pankaj Raghav <p.raghav@samsung.com>
Cc: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>