NFSv4/pnfs: Clean up layout get on open

Cache the layout in the arguments so we don't have to keep looking it up
from the inode.

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>

include/linux/nfs_xdr.h

@@ -277,6 +277,7 @@ struct nfs4_layoutget {
 	struct nfs4_layoutget_args args;
 	struct nfs4_layoutget_res res;
 	const struct cred *cred;
+	struct pnfs_layout_hdr *lo;
 	gfp_t gfp_flags;
 };