buffer: add get_nth_bh()

Extract this useful helper from nilfs_page_get_nth_block()

Link: https://lkml.kernel.org/r/20231016201114.1928083-6-willy@infradead.org


Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Acked-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Cc: Andreas Gruenbacher <agruenba@redhat.com>
Cc: Pankaj Raghav <p.raghav@samsung.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>