nfsd: Make the pNFS server update Change for Setxattr/Rmxattr

When the NFS server does the Setxattr or Rmxattr operation,
the Change attribute (va_filerev) needs to be updated.

Without this patch, that was not happening for the
pNFS server configuration. This patch does a Setattr
against the DS file to make the Change attribute
change.

This bug was discovered during a recent IETF NFSv4 testing
event, where the Change attribute wasn't changed in the
operation reply.

MFC after: 1 month