NFSv4/pnfs: minor fix for cleanup path in nfs4_get_device_info

It is an almost improbable error case but when page allocating loop in
nfs4_get_device_info() fails then we should only free the already
allocated pages, as __free_page() can't deal with NULL arguments.

Found by Linux Verification Center (linuxtesting.org).

Cc: stable@vger.kernel.org
Signed-off-by: Fedor Pchelkin <pchelkin@ispras.ru>
Reviewed-by: Benjamin Coddington <bcodding@redhat.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>