btrfs: collapse __btrfs_qgroup_free_meta() into btrfs_qgroup_free_meta_prealloc()
Since __btrfs_qgroup_free_meta() is only called by
btrfs_qgroup_free_meta_prealloc(), which is a simple inline wrapper, get
rid of the later and rename __btrfs_qgroup_free_meta() to the later.
Reviewed-by: Qu Wenruo <wqu@suse.com> Signed-off-by: Filipe Manana <fdmanana@suse.com> Reviewed-by: David Sterba <dsterba@suse.com> Signed-off-by: David Sterba <dsterba@suse.com>