Btrfs: fix wrong super generation mismatch when scrubbing supers

We came a race condition when scrubbing superblocks, the story is:

In commiting transaction, we will update @last_trans_commited after
writting superblocks, if scrubber start after writting superblocks
and before updating @last_trans_commited, generation mismatch happens!

We fix this by checking @scrub_pause_req, and we won't start a srubber
until commiting transaction is finished.(after btrfs_scrub_continue()
finished.)

Reported-by: Sebastian Ochmann <ochmann@informatik.uni-bonn.de>
Signed-off-by: Wang Shilong <wangsl.fnst@cn.fujitsu.com>
Reviewed-by: Miao Xie <miaox@cn.fujitsu.com>
Signed-off-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Chris Mason <clm@fb.com>