ext4: convert mext_page_double_lock() to mext_folio_double_lock()

Convert mext_page_double_lock() to use folios.  This change saves 146
bytes of kernel text.  It also removes 6 calls to compound_head() and 2
calls to folio_file_page().

Link: https://lkml.kernel.org/r/20221207181009.4016-1-vishal.moola@gmail.com


Signed-off-by: Vishal Moola (Oracle) <vishal.moola@gmail.com>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>