ubifs: ubifs_jnl_change_xattr: Remove assertion 'nlink > 0' for host inode

Changing xattr of a temp file will trigger following assertion failed
and make ubifs turn into readonly filesystem:
  ubifs_assert_failed [ubifs]: UBIFS assert failed: host->i_nlink > 0,
  in fs/ubifs/journal.c:1801

Reproducer:
  1. fd = open(__O_TMPFILE)
  2. fsetxattr(fd, key, value2, XATTR_CREATE)
  3. fsetxattr(fd, key, value2, XATTR_REPLACE)

Fix this by removing assertion 'nlink > 0' for host inode.

Reported-by: Chengsong Ke <kechengsong@huawei.com>
Signed-off-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>