nfsserver: Fix handling of SP4_NONE

For NFSv4.1/4.2, when the client specifies SP4_NONE for
state protection in the ExchangeID operation arguments,
the server MUST allow the state management operations for
any user credentials. (I misread the RFC and thought that
SP4_NONE meant "at the server's discression" and not MUST
be allowed.)

This means that the "sec=XXX" field of the "V4:" exports(5)
line only applies to NFSv4.0.

This patch fixes the server to always allow state management
operations for SP4_NONE, which is the only state management
option currently supported. (I have patches that add support
for SP4_MACH_CRED to the server. These will be in a future commit.)

In practice, this bug does not seem to have caused
interoperability problems.

MFC after: 2 weeks