btrfs: sink gfp_t parameter to alloc_scrub_sector

All callers pas GFP_KERNEL as parameter so we can use it directly in
alloc_scrub_sector.

Reviewed-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: David Sterba <dsterba@suse.com>