pNFS: Handle allocation errors correctly in filelayout_alloc_layout_hdr()

commit 6df200f5d5191bdde4d2e408215383890f956781 upstream.

Return the NULL pointer when the allocation fails.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>