btrfs: fix misleading variable name for flags

The variable was named 'data' in btrfs_reserve_extent and that's the
only function that actually uses it to let btrfs_get_alloc_profile know
what profile we want. Then it's passed down as u64 flags.

Signed-off-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>