smb/client: mark missing nlink values as unknown

Several SMB1 fallback and open responses do not provide the hard link
count. The SMB2 create-only query fallback has the same limitation.
These paths currently leave a zero link count or synthesize a value of
one and then expose it as authoritative metadata.

Mark those results with unknown_nlink so existing inodes keep their
cached link count and new inodes receive the usual sane default.

This was tested against Samba with "server min protocol = NT1". Mount
the share using SMB1 with Unix extensions disabled:

  mount -t cifs //<server>/<share> /mnt/cifs \
        -o username=<user>,vers=1.0,nounix

Create three names for the same inode and cache its real link count:

  TESTDIR=/mnt/cifs/nlink-repro-$$
  mkdir "$TESTDIR"
  touch "$TESTDIR/file1"
  ln "$TESTDIR/file1" "$TESTDIR/file2"
  ln "$TESTDIR/file1" "$TESTDIR/file3"
  stat -c 'before open: %h' "$TESTDIR/file1"

Open the file and read the link count through the open descriptor:

  exec 3<"$TESTDIR/file1"
  stat -Lc 'after open: %h' /proc/$$/fd/3
  exec 3<&-

Clean up the test files:

  rm -f "$TESTDIR/file1" "$TESTDIR/file2" "$TESTDIR/file3"
  rmdir "$TESTDIR"

Before this change, the two stat commands report 3 and 1 because the
SMB1 open response overwrites the known link count. With this change,
both commands report 3.

Signed-off-by: Ze Tan <tanze@kylinos.cn>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Paulo Alcantara <pc@manguebit.org>