alloc: improve estimation of sufficient_pes_free

alloc: improve estimation of sufficient_pes_free

Metadata size was calculated correctly only for raids.

Fixes problem for crash during lvcreate when thin-pool was created
on a VG where remaining free space had the size to only fit a single
metadata LV and not also its _pmspare.

Lvcreate crashed with this assert message:

lvcreate: metadata/pv_map.c:198: consume_pv_area: Assertion `to_go <= pva->count' failed.
Aborted (core dumped)

TODO: there is probably to large overload of several alloc_handle
variables.

Reported-by: Wu Guanghao<wuguanghao3@huawei.com>
Reported-by: Zhiqiang Liu <liuzhiqiang26@huawei.com>