ext4: convert ext4_mb_init_cache() to take a folio

All callers now have a folio, so convert this function from operating on
a page to operating on a folio.  The folio is assumed to be a single page.

Signe-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Link: https://lore.kernel.org/r/20240416172900.244637-4-willy@infradead.org


Signed-off-by: Theodore Ts'o <tytso@mit.edu>