smb: client: handle STATUS_STOPPED_ON_SYMLINK responses without a symlink target

smb: client: handle STATUS_STOPPED_ON_SYMLINK responses without a symlink target

The macOS built-in SMB server returns STATUS_STOPPED_ON_SYMLINK for a
CREATE on a path whose final component is a symlink, but it does not
include a Symbolic Link Error Response in the error data: both
ErrorContextCount and ByteCount are zero, so the symlink target is not
present in the response at all.  Per [MS-SMB2] section 2.2.2 such a
response should carry a valid Symbolic Link Error Response, so this is a
server bug, but the target can still be retrieved with
FSCTL_GET_REPARSE_POINT.

Frame from a capture against macOS 26.5.2 (build 25F84):

    SMB2 hdr : Status=0x8000002d STATUS_STOPPED_ON_SYMLINK, Cmd=Create
    Error Rsp: StructureSize=0x0009
               Error Context Count: 0
               Byte Count: 0
               Error Data: 00

symlink_data() cannot find a struct smb2_symlink_err_rsp in such a
response and returns -EINVAL, which parse_create_response() propagates,
so smb2_query_path_info() bails out at

	if (rc || !data->reparse_point)
		goto out;

before it can retry with SMB2_OP_GET_REPARSE.  stat(), readlink() and ls
of any server-side symlink then fail with -EINVAL:

  $ ls -la Config
  l????????? ? ? ? ? ? Config.json
  $ stat Config/Config.json
  stat: cannot statx 'Config/Config.json': Invalid argument

A 5.10 client resolves these symlinks correctly against the same server
and share, so this is a regression for Apple SMB servers.

Handle it in several places:

 - symlink_data() detects the empty response (ErrorContextCount and
   ByteCount both zero) and returns a distinct -ENODATA, so that "server
   did not send the target" can be told apart from a genuinely malformed
   response and only this case is worked around.

 - parse_create_response() treats -ENODATA like
   STATUS_IO_REPARSE_TAG_NOT_HANDLED, which does not carry the target
   either: leave the reparse tag unset and clear rc, so the existing
   SMB2_OP_GET_REPARSE path retrieves the target.

 - smb2_query_path_info() only fixes up the symlink target type when the
   target is already known.  SMB2_OP_GET_REPARSE sets data->reparse.tag
   but does not parse the target out of the reparse buffer; that happens
   later, in reparse_info_to_fattr().  Without this check
   smb2_fix_symlink_target_type() is called with a NULL target and
   returns -EIO.  This could not happen with servers that send the target
   inline and therefore skip SMB2_OP_GET_REPARSE.

 - smb2_open_file() maps -ENODATA to -EIO, matching
   STATUS_IO_REPARSE_TAG_NOT_HANDLED, so its callers retrieve the target
   with SMB2_OP_GET_REPARSE as well.

Tested on Debian 13, kernel 6.18.38 (armv7), against macOS 26.5.2:
symlinks now resolve, including relative, parent-traversing and directory
symlinks, and reads through symlinks succeed.

Cc: stable@vger.kernel.org
Co-developed-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Carl Johnson <carl@jpartners.org>
Signed-off-by: Steve French <stfrench@microsoft.com>