[SMB3] Fix oops when creating symlinks on smb3

We were not checking for symlink support properly for SMB2/SMB3
mounts so could oops when mounted with mfsymlinks when try
to create symlink when mfsymlinks on smb2/smb3 mounts

Signed-off-by: Steve French <smfrench@gmail.com>
Cc: <stable@vger.kernel.org> # 3.14+
CC: Sachin Prabhu <sprabhu@redhat.com>