ext4: move keep_towrite handling to ext4_bio_write_page()

ext4: move keep_towrite handling to ext4_bio_write_page()

When we are writing back page but we cannot for some reason write all
its buffers (e.g. because we cannot allocate blocks in current context) we
have to keep TOWRITE tag set in the mapping as otherwise racing
WB_SYNC_ALL writeback that could write these buffers can skip the page
and result in data loss.  We will need this logic for writeback during
transaction commit so move the logic from ext4_writepage() to
ext4_bio_write_page().

Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20221207112722.22220-2-jack@suse.cz
Signed-off-by: Theodore Ts'o <tytso@mit.edu>