btrfs: scrub: factor out allocation and initialization of scrub_sector into helper

The allocation and initialization is shared by 3 call sites, and we're
going to change the initialization of some members in the upcoming
patches.

So factor out the allocation and initialization of scrub_sector into a
helper, alloc_scrub_sector(), which will do the following work:

- Allocate the memory for scrub_sector

- Allocate a page for scrub_sector::page

- Initialize scrub_sector::refs to 1

- Attach the allocated scrub_sector to scrub_block
  The attachment is bidirectional, which means scrub_block::sectorv[]
  will be updated and scrub_sector::sblock will also be updated.

- Update scrub_block::sector_count and do extra sanity check on it

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