fs_mgr: CreateDmTable takes CreateLogicalPartitionParams

It has too many arguments.

Also fixed CreateLogicalPartitionParams::InitDefaults because
it doesn't use the provided partition opener to read metadata
(which breaks tests).

Test: libsnapshot_test
Test: liblp_test

Change-Id: I74cf8d468274f741c6f3743438fe8855b2aef15b