ubifs: Don't add xattr inode into orphan area

Now, the entire inode with its' xattrs are removed while replaying
orphan nodes. There is no need to add xattr inodes into orphan area,
which is based on the fact that xattr entries won't be cleared from
disk before deleting xattr inodes, in another words, current logic
can make sure that xattr inode be deleted in any cases even UBIFS not
record xattr inode into orphan area.
Let's looking for possible paths that could clear xattr entries from
disk but leave the xattr inode on TNC:
 1. unlink/tmpfile -> ubifs_jnl_update: inode(nlink=0) is written
    into bud LEB and added into orphan list, then:
    a. powercut: ubifs_tnc_remove_ino(xattr entry/inode can be found
       from TNC and being deleted) is invoked in replaying journal.
    b. commit + powercut: inode is written into orphan area, and
       ubifs_tnc_remove_ino is invoked in replaying orphan nodes.
    c. evicting + powercut: xattr inode(nlink=0) is written on disk,
       xattr is removed from TNC, gc could clear xattr entries from
       disk. ubifs_tnc_remove_ino will apply on inode and xattr inode
       in replaying journal, so lost xattr entries will make no
       influence.
    d. evicting + commit + powercut: xattr inode/entry are removed from
       index tree(on disk) by ubifs_jnl_write_inode, xattr inode is
       cleared from orphan area by ubifs_jnl_write_inode + commit.
    e. commit + evicting + powercut: inode is written into orphan area,
       then equivalent to c.
 2. remove xattr -> ubifs_jnl_delete_xattr: xattr entry(inum=0) and
    xattr inode(nlink=0) is written into bud LEB, xattr entry/inode are
    removed from TNC, then:
    a. powercut: gc could clear xattr entries from disk, which won't
       affect deleting xattr entry from TNC. ubifs_tnc_remove_ino will
       apply on xattr inode in replaying journal, ubifs_tnc_remove_nm
       will apply on xattr entry in replaying journal.
    b. commit + powercut: xattr entry/inode are removed from index tree
       (on disk).
Tracking xattr inode in orphan list is imported by commit 988bec41
("ubifs: orphan: Handle xattrs like files"), it aims to fix the similar
problem described in commit 7959cf3a

 ("ubifs: journal: Handle
xattrs like files"). Actually, the problem only exist in journal case
but not the orphan case. So, we can remove the orphan tracking for xattr
inodes.

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