Rename 'btrfs_io_stripe::is_scrub' to 'rst_search_commit_root'. While
'is_scrub' describes the state of the io_stripe (it is a stripe submitted
by scrub) it does not describe the purpose, namely looking at the commit
root when searching RAID stripe-tree entries.

Renaming the stripe to rst_search_commit_root describes this purpose.
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>