e2fsck: eliminate bad update link count attempt

e2fsck: eliminate bad update link count attempt

Extended attribute inodes have a link count of 1 but they are not
attached to any directories. When an xattr inode with zero ea
references is found, the remedy is to reconnect it to lost+found dir.
Since reconnect operation increments the link count, it would normally
become 2 but to avoid that, check_ea_inode() sets the link count to
zero in anticipation of reconnect operation. And it does it even when
e2fsck is invoked with -n option which causes a fatal e2fsck failure
as can be demonstrated with the following test script:

  mke2fs -b 4096 -I 256 -F -O extent,ea_inode /dev/sdb 1G
  mount /dev/sdb /mnt/sdb
  touch /mnt/sdb/x
  setfattr -n user.ie1 -v $(python -c "print 'v'*5000") /mnt/sdb/x
  umount /dev/sdb
  debugfs -w -R 'rm x' /dev/sdb
  e2fsck -f -n /dev/sdb

In this case, e2fsck fails with the following error:

  e2fsck 1.43.5-WIP (17-Feb-2017)
  Pass 1: Checking inodes, blocks, and sizes
  Pass 2: Checking directory structure
  Pass 3: Checking directory connectivity
  Pass 4: Checking reference counts
  ext2fs_write_inode: Attempt to write to filesystem opened read-only
  while writing inode 13 in check_ea_inode
  e2fsck: aborted

Setting link count to zero is not really required because e2fsck will
detect the discrepancy in link count and offer to fix it.

Signed-off-by: Tahsin Erdogan <tahsin@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>