NFSD fix mismatching type in nfsd4_set_netaddr

Fix __be32 and u32 mismatch in return and assignment.

Reported-by: kbuild test robot <lkp@intel.com>
Fixes: dbd4c2dd8f13 ("NFSD add COPY_NOTIFY operation")
Signed-off-by: Olga Kornievskaia <kolga@netapp.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>