nilfs2: convert nilfs_find_entry to use a folio

Use the new folio APIs to remove calls to compound_head().

[konishi.ryusuke: resolved a conflict due to style warning correction]
Link: https://lkml.kernel.org/r/20231127143036.2425-12-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>