nfsd4: fix v3-less build

Includes of pnfs.h in export.c and fcntl.c also bring in xdr4.h, which
won't build without CONFIG_NFSD_V3, breaking non-V3 builds.  Ifdef-out
most of pnfs.h in that case.

Reported-by: Bas Peters <baspeters93@gmail.com>
Reported-by: Jim Davis <jim.epost@gmail.com>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Fixes: 9cf514cc "nfsd: implement pNFS operations"
Signed-off-by: J. Bruce Fields <bfields@redhat.com>