buffer: fix more functions for block size > PAGE_SIZE

Both __block_write_full_folio() and block_read_full_folio() assumed that
block size <= PAGE_SIZE.  Replace the shift with a divide, which is
probably cheaper than first calculating the shift.  That lets us remove
block_size_bits() as these were the last callers.

Link: https://lkml.kernel.org/r/20231109210608.2252323-8-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>