pNFS/flexfiles: missing error code in ff_layout_alloc_lseg()

If xdr_inline_decode() fails then we end up returning ERR_PTR(0).  The
caller treats NULL returns as -ENOMEM so it doesn't really hurt runtime,
but obviously we intended to set an error code here.

Fixes: d67ae825 ("pnfs/flexfiles: Add the FlexFile Layout Driver")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>