nfsd4: store correct client minorversion for >=4.2

This code assumes that any client using exchange_id is using NFSv4.1,
but with the introduction of 4.2 that will no longer true.

This main effect of this is that client callbacks will use the same
minorversion as that used on the exchange_id.

Note that clients are forbidden from mixing 4.1 and 4.2 compounds.  (See
rfc 5661, section 2.7, #13: "A client MUST NOT attempt to use a stateid,
filehandle, or similar returned object from the COMPOUND procedure with
minor version X for another COMPOUND procedure with minor version Y,
where X != Y.")  However, we do not currently attempt to enforce this
except in the case of mixing zero minor version with non-zero minor
versions.

Signed-off-by: J. Bruce Fields <bfields@redhat.com>