btrfs: sink gfp_t parameter to btrfs_backref_iter_alloc

There's only one caller that passes GFP_NOFS, we can drop the parameter
an use the flags directly.

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