cifs: Use a folio in cifs_page_mkwrite()

Avoids many calls to compound_head() and removes calls to various
compat functions.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: David Howells <dhowells@redhat.com>
Signed-off-by: Steve French <stfrench@microsoft.com>