nfsd4: buffer-length check for SUPPATTR_EXCLCREAT

This was an omission from 8c18f205


"nfsd41: SUPPATTR_EXCLCREAT attribute".

Cc: Benny Halevy <bhalevy@primarydata.com>
Cc: stable@vger.kernel.org
Signed-off-by: J. Bruce Fields <bfields@redhat.com>