cifs: spnego: add ';' in HOST_KEY_LEN

"host=" should start with ';' (as in cifs_get_spnego_key)
So its length should be 6.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Reviewed-by: Paulo Alcantara (SUSE) <pc@manguebit.com>
Fixes: 7c9c3760 ("[CIFS] add constants for string lengths of keynames in SPNEGO upcall string")
Signed-off-by: Anastasia Belova <abelova@astralinux.ru>
Co-developed-by: Ekaterina Esina <eesina@astralinux.ru>
Signed-off-by: Ekaterina Esina <eesina@astralinux.ru>
Signed-off-by: Steve French <stfrench@microsoft.com>