Fix remote metadata backup for clvmd

Fix remote metadata backup for clvmd

Run backup of metadata on remote nodes in the
same place like local node - when calling backup().

Introduce backup_locally() which calls only
local backup if needed.

Remote backup is now trigerred by LCK_VG_BACKUP flag
combination (special VG lock).

This lock type will call check_current_backup()
(including backup_locally() call) and updates
metadata on all nodes.

(Patch fixes non-functional remote backup,
current call during VG lock never triggers.)