nfsd4: reserve space before inlining 0-copy pages

Once we've included page-cache pages in the encoding it's difficult to
remove them and restart encoding.  (xdr_truncate_encode doesn't handle
that case.)  So, make sure we'll have adequate space to finish the
operation first.

For now COMPOUND_SLACK_SPACE checks should prevent this case happening,
but we want to remove those checks.

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