nilfs2: use set_mask_bits() for operations on buffer state bitmap

nilfs2: use set_mask_bits() for operations on buffer state bitmap

nilfs_forget_buffer(), nilfs_clear_dirty_page(), and
nilfs_segctor_complete_write() are using a bunch of atomic bit operations
against buffer state bitmap.

This reduces the number of them by utilizing set_mask_bits() macro.

Signed-off-by: Ryusuke Konishi <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>