pNFS: Handle allocation errors correctly in filelayout_alloc_layout_hdr()

Return the NULL pointer when the allocation fails.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Cc: <stable@vger.kernel.org> # 3.5.x
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>