Since the case where secflavor < 0 indicates the security flavor is
to be negotiated, it could be a Kerberized mount. As such, filling
in the "principal" argument using the canonized host name makes sense.
If it is negotiated as AUTH_SYS, the "principal" argument is meaningless
but harmless.

Requested by: masato@itc.naist.jp
Tested by: masato@itc.naist.jp
PR: 201073
MFC after: 1 month