btrfs: scrub: use pointer array to replace sblocks_for_recheck

In function scrub_handle_errored_block(), we use @sblocks_for_recheck
pointer to hold one scrub_block for each mirror, and uses kcalloc() to
allocate an array.

But this one pointer for an array is not readable due to the member
offsets done by addition and not [].

Change this pointer to struct scrub_block *[BTRFS_MAX_MIRRORS], this
will slightly increase the stack memory usage.

Since function scrub_handle_errored_block() won't get iterative calls,
this extra cost would completely be acceptable.

And since we're here, also set sblock->refs and use scrub_block_put() to
clean them up, as later we will add extra members in scrub_block, which
needs scrub_block_put() to clean them up.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>