gfs2: convert inode unstuffing to use a folio

Use the folio APIs, removing numerous hidden calls to compound_head(). 
Also remove the stale comment about the page being looked up if it's NULL.

Link: https://lkml.kernel.org/r/20231016201114.1928083-7-willy@infradead.org


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