ext4: convert write_begin methods to stable_page_writes semantics

Use wait_for_stable_page() instead of wait_on_page_writeback()

Signed-off-by: Dmitry Monakhov <dmonakhov@openvz.org>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Reviewed-by: Jan Kara <jack@suse.cz>