libdm: file: add proper checks for directory components in dm_create_dir

libdm: file: add proper checks for directory components in dm_create_dir

Also make error messages more consistent:

Before this patch:

(/run/lock exists and is not a directory)
$ pvs
  /run/lock/lvm: mkdir failed: Not a directory
  File-based locking initialisation failed.

(/run/lock/lvm exists and is not a directory)
$ pvs
  Directory "/run/lock/lvm" not found
  File-based locking initialisation failed.

With this patch applied:

(/run/lock exists and is not a directory)
$ pvs
  Existing path /run/lock is not a directory.
  Failed to create directory /run/lock/lvm.
  File-based locking initialisation failed

(/run/lock/lvm exists and is not a directory)
$ pvs
  Existing path /run/lock/lvm is not a directory.
  Failed to create directory /run/lock/lvm.
  File-based locking initialisation failed.