btrfs: rename btrfs_get_block_group_info and make it static

The function btrfs_get_block_group_info() was introduced by the
commit 5af3e8cc

 ("Btrfs: make filesystem read-only when submitting
 barrier fails") which used it in disk-io.c.

However, the function is only called in ioctl.c now.
Its parameter type btrfs_ioctl_space_info* is only for ioctl.

So, make it static and rename it to be original name
get_block_group_info.

No functional change.

Signed-off-by: Su Yue <suy.fnst@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.com>