libmount: don't treat "none" differently

libmount: don't treat "none" differently

This causes more problems than it solves. In the latest edition:

  # mount -t proc none foo
  mount: foo: mount failed: Invalid argument

A check for source and target fails in mnt_context_apply_fstab()
because, even though they were indeed specified on the cmdline,
__mnt_fs_set_source_ptr() altered this and NULL'd out the source.

If you're able to mount this device via other means, other tools start
reporting oddities, such as mount's output:

  (null) on /foo type proc (rw,relatime)

or findmnt:

  TARGET      SOURCE FSTYPE OPTIONS
  /foo               proc   rw,relatime

Simply treat "none" like any other source when passed in.

[kzak@redhat.com: - don't translate NULL to "none" in mnt_fs_set_source()]

Signed-off-by: Dave Reisner <dreisner@archlinux.org>
Signed-off-by: Karel Zak <kzak@redhat.com>