md/bitmap: don't read page from device with Bitmap_sync

The device owns Bitmap_sync flag needs recovery
to become in sync, and read page from this type
device could get stale status.

Also add comments for Bitmap_sync bit per the
suggestion from Shaohua and Neil.

Previous disscussion can be found here:
https://marc.info/?t=149760428900004&r=1&w=2



Signed-off-by: Guoqing Jiang <gqjiang@suse.com>
Signed-off-by: Shaohua Li <shli@fb.com>