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libmount: Fix regression when mounting with atime

A regression was introduced in v2.39 that causes mounting with the atime
option to fail:

  $ mkfs.ext4 -F /dev/sdi
  $ mount -o atime /dev/sdi /mnt/sdi
  mount: /mnt/sdi: not mount point or bad option.
         dmesg(1) may have more information after failed mount system call.

The failure comes from the mount_setattr(2) call returning -EINVAL. This
is because we pass an invalid value for the attr_clr argument. From a
strace capture we have:

  mount_setattr(4, "", AT_EMPTY_PATH, {attr_set=0, attr_clr=MOUNT_ATTR_NOATIME, propagation=0 /* MS_??? */, userns_fd=0}, 32) = -1 EINVAL (Invalid argument)

We can't pass MOUNT_ATTR_NOATIME to mount_setattr(2) through the attr_clr
argument because all atime options are exclusive, so in order to set atime
one has to pass MOUNT_ATTR__ATIME to attr_clr and leave attr_set as
MOUNT_ATTR_RELATIME (which is defined as a value of 0).

This can be read from the man page for mount_setattr(2) and also from the
kernel source:

  $ cat fs/namespace.c
  static int build_mount_kattr(const struct mount_attr *attr, size_t usize,
                               struct mount_kattr *kattr, unsigned int flags)
  {
      (...)
      /*
       * Since the MOUNT_ATTR_<atime> values are an enum, not a bitmap,
       * users wanting to transition to a different atime setting cannot
       * simply specify the atime setting in @attr_set, but must also
       * specify MOUNT_ATTR__ATIME in the @attr_clr field.
       * So ensure that MOUNT_ATTR__ATIME can't be partially set in
       * @attr_clr and that @attr_set can't have any atime bits set if
       * MOUNT_ATTR__ATIME isn't set in @attr_clr.
       */
      if (attr->attr_clr & MOUNT_ATTR__ATIME) {
          if ((attr->attr_clr & MOUNT_ATTR__ATIME) != MOUNT_ATTR__ATIME)
              return -EINVAL;

              /*
               * Clear all previous time settings as they are mutually
               * exclusive.
               */
  ... (continued)

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