A pNFS server may return LAYOUTUNAVAILABLE error on LAYOUTGET for files
which don't have any layout. In this situation pnfs_update_layout
currently returns NULL. As this NULL is converted into ENOMEM, IO
requests fails instead of falling back to MDS.

Do not return ENOMEM on LAYOUTUNAVAILABLE and let client retry through
MDS.

Fixes 8d40b0f1. I will suggest to backport this fix to affected
stable branches.
Signed-off-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
[trondmy: Use IS_ERR_OR_NULL()]
Fixes: 8d40b0f1 ("NFS filelayout:call GETDEVICEINFO after...")
Cc: stable@vger.kernel.org # v4.11+
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>