lvcreate: check for lv type and created segtype

lvcreate: check for lv type and created segtype

When creating LV - in some case we change created segment type
(ATM for cache and snapshot) and we then manipulate with
lv segment according to 'lp' segtype.
Fix this by checking for proper type before accessing segment members.

This makes command like:

lvcreate --type cache-pool -L10 vg/cpool
lvcreate -H -L10 --cachesettings migtation_threshold=10000  vg/cpool

to pass since now tool correctly selects default cache policy.