mount: use get_mount_parameters_fragment() consistently

mount: use get_mount_parameters_fragment() consistently

There are multiple different checks, that all mean the same thing:
Is it a explicitly configured mount unit where actions need to be taken to
mount it, or is is just mirroring 'mountinfo':
'from_fragment' to set if fragment_path is not NULL, and
get_mount_parameters_fragment() just wraps that and returns fragment_path.

Use get_mount_parameters_fragment() everywhere to be consistent.
This is just a cleanup without functional change.