NFS: Fix a use-before-initialised warning in fs/nfs/write.c and fs/nfs/pnfs.c

If the allocation of nfs_write_header fails, the list of nfs_pages that
needs to be cleaned up is still on desc->pg_list...

Reported-by: Bryan Schumaker <bjschuma@netapp.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Acked-by: Fred Isaman <iisaman@netapp.com>