btrfs: rename btrfs_io_stripe::is_scrub to rst_search_commit_root

btrfs: rename btrfs_io_stripe::is_scrub to rst_search_commit_root

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 <[email protected]>
Reviewed-by: Qu Wenruo <[email protected]>
Signed-off-by: Johannes Thumshirn <[email protected]>
Reviewed-by: David Sterba <[email protected]>
Signed-off-by: David Sterba <[email protected]>