UBIFS: use bit-fields when possible

The "bulk_read" and "no_chk_data_crc" have only 2 values -
0 and 1. We already have bit-fields in corresponding data
structers, so make "bulk_read" and "no_chk_data_crc"
bit-fields as well.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>