libmount: fix mnt_table_is_fs_mounted() for NFS bind mounts.
When you bind-mount a subdirectory of a local filesystem, the
path to that subdirectory appears as the fourth field in mountinfo.
For nfs mounts, the fourth field is always "/", and the subdirectory
part is appended to the "special" (aka "device") field. This is
consistent with historical NFS usage which always includes a path in
the fs_spec field.
libmount needs to know about this when "mount -a" checks to see if
a filesystem is already mounted.