md/raid1: remove unnecessary BUG_ON

Since bitmap_start_sync will not return until
sync_blocks is not less than PAGE_SIZE>>9, so
the BUG_ON is not needed anymore.

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