nilfs2: convert nilfs_btnode_prepare_change_key to use a folio

Saves three calls to compound_head().

Link: https://lkml.kernel.org/r/20231114084436.2755-19-konishi.ryusuke@gmail.com


Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>