pNFS: Handle allocation errors correctly in filelayout_alloc_layout_hdr()

Return the NULL pointer when the allocation fails.

Reported-by: Fengguang Wu <[e-mail]>
Cc: <[e-mail]> # 3.5.x
Signed-off-by: Trond Myklebust <[e-mail]>

fs/nfs/nfs4filelayout.c

···
 	struct nfs4_filelayout *flo;
 
 	flo = kzalloc(sizeof(*flo), gfp_flags);
-	return &flo->generic_hdr;
+	return flo != NULL ? &flo->generic_hdr : NULL;
 }
 
 static void