btrfs: collapse __btrfs_qgroup_reserve_meta() into btrfs_qgroup_reserve_meta_prealloc()
Since __btrfs_qgroup_reserve_meta() is only called by
btrfs_qgroup_reserve_meta_prealloc(), which is a simple inline wrapper,
get rid of the later and rename __btrfs_qgroup_reserve_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>