NFSv4/pnfs: Handle RPC allocation errors in nfs4_proc_layoutget

If rpc_run_task() fails due to an allocation error, then bail out early.

Fixes: 910ad386

 ("NFS: Fix memory allocation in rpc_alloc_task()")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>