[JFFS2] Fix BUG() caused by failing to discard xattrs on deleted files.

When we cannot mark nodes as obsolete, such as on NAND flash, we end up 
having to delete inodes with !nlink in jffs2_build_remove_unlinked_inode().
However, jffs2_build_xattr_subsystem() runs later than this, and will
attach an xref to the dead inode. Then later when the last nodes of that
dead inode are erased we hit a BUG() in jffs2_del_ino_cache() 
because we're not supposed to get there with an xattr still attached to 
the inode which is being killed.

The simple fix is to refrain from attaching xattrs to inodes with zero 
nlink, in jffs2_build_xattr_subsystem(). It's it's OK to trust nlink 
here because the file system isn't actually mounted yet, so there's no 
chance that a zero-nlink file could actually be alive still because 
it's open.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>