Record `m_sb` and `m_dif` to replace `m_fscache`, `m_daxdev`, `m_fp`
and `m_dax_part_off` in order to simplify the codebase.
Note that `m_bdev` is still left since it can be assigned from
`sb->s_bdev` directly.
Signed-off-by: Gao Xiang <hsiangkao@xxxxxxxxxxxxxxxxx>