btrfs: Refactor __get_raid_index() to btrfs_bg_flags_to_raid_index()

Function __get_raid_index() is used to convert block group flags into
raid index, which can be used to get various info directly from
btrfs_raid_array[].

Refactor this function a little:

1) Rename to btrfs_bg_flags_to_raid_index()
   Double underline prefix is normally for internal functions, while the
   function is used by both extent-tree and volumes.

   Although the name is a little longer, but it should explain its usage
   quite well.

2) Move it to volumes.h and make it static inline
   Just several if-else branches, really no need to define it as a normal
   function.

   This also makes later code re-use between kernel and btrfs-progs
   easier.

3) Remove function get_block_group_index()
   Really no need to do such a simple thing as an exported function.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>