Btrfs-progs: add option to skip whether a scrub has started/resumed in userspace

Btrfs-progs: add option to skip whether a scrub has started/resumed in userspace

I hit a problem that i can not start scrub when i am trying to track
superblock generation mismatch problems.

The fact is that we are trying to check whether we have started a scrub operation
in userspace, this will make us can't start scrub if that record file is damaged
itself. By adding a option to skip that check, everything will be fine.

Signed-off-by: Wang Shilong <wangsl.fnst@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Chris Mason <clm@fb.com>