umount: Don't use options from fstab on remount

umount: Don't use options from fstab on remount

The fstab entry may contain comment/application-specific options, like
for example x-systemd.automount or x-initrd.mount.

With the recent switch to libmount, the mount options during remount are
now gathered via mnt_fs_get_options(), which returns the merged fstab
options with the effective options in mountinfo.

Unfortunately if one of these application-specific options are set in
fstab, the remount will fail with -EINVAL.

In systemd 238:

  Remounting '/test-x-initrd-mount' read-only in with options
  'errors=continue,user_xattr,acl'.

In systemd 239:

  Remounting '/test-x-initrd-mount' read-only in with options
  'errors=continue,user_xattr,acl,x-initrd.mount'.
  Failed to remount '/test-x-initrd-mount' read-only: Invalid argument

So instead of using mnt_fs_get_options(), we're now using both
mnt_fs_get_fs_options() and mnt_fs_get_vfs_options() and merging the
results together so we don't get any non-relevant options from fstab.

Signed-off-by: aszlig <aszlig@nix.build>