ext4: Use scoped memory API in mext_page_double_lock()

Replace use of AOP_FLAG_NOFS with calls to memalloc_nofs_save()
and memalloc_nofs_restore().

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Acked-by: Theodore Ts'o <tytso@mit.edu>