mount: apply "nofail" to MNT_ERR_NOSOURCE libmount error

fstab:
     UUID=nonexist /mnt/nonexist1 ext4 nofail 0 1

 # mount -av
 mount: can't find UUID=nonexist

.. this is bug of course.

Reported-by: Patrick McLean <chutzpah@gentoo.org>
Signed-off-by: Karel Zak <kzak@redhat.com>

libmount/src/context.c

@@ -716,6 +716,11 @@ void *mnt_context_get_mtab_userdata(struct libmnt_context *cxt)
  * @cxt: mount context
  * @source: mount source (device, directory, UUID, LABEL, ...)
  *
+ * Note that libmount does not interpret "nofail" (MNT_MS_NOFAIL)
+ * mount option. The real return code is always returned, when
+ * the device does not exist then it's usually MNT_ERR_NOSOURCE
+ * from libmount or ENOENT, ENOTDIR, ENOTBLK, ENXIO from moun(2).
+ *
  * Returns: 0 on success, negative number in case of error.
  */
 int mnt_context_set_source(struct libmnt_context *cxt, const char *source)

sys-utils/mount.c

@@ -431,6 +431,8 @@ try_readonly:
 				warnx(_("you must specify the filesystem type"));
 			return MOUNT_EX_USAGE;
 		case -MNT_ERR_NOSOURCE:
+			if (uflags & MNT_MS_NOFAIL)
+				return MOUNT_EX_SUCCESS;
 			if (src)
 				warnx(_("can't find %s"), src);
 			else