ext4: Use a folio in ext4_read_merkle_tree_page

This is an implementation of fsverity_operations read_merkle_tree_page,
so it must still return the precise page asked for, but we can use the
folio API to reduce the number of conversions between folios & pages.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Link: https://lore.kernel.org/r/20230324180129.1220691-30-willy@infradead.org


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