NFS: Add missing null check for failed allocation

Currently the allocation of buf is not being null checked and
a null pointer dereference can occur when the memory allocation fails.
Fix this by adding a check and returning -ENOMEM.

Addresses-Coverity: ("Dereference null return")
Fixes: 6d972518b821 ("NFS: Add fs_context support.")
Signed-off-by: NColin Ian King <colin.king@canonical.com>
Signed-off-by: NAnna Schumaker <Anna.Schumaker@Netapp.com>

fs/nfs/nfs4namespace.c

@@ -137,6 +137,9 @@ static int nfs4_validate_fspath(struct dentry *dentry,
 	int n;
 
 	buf = kmalloc(4096, GFP_KERNEL);
+	if (!buf)
+		return -ENOMEM;
+
 	path = nfs4_path(dentry, buf, 4096);
 	if (IS_ERR(path)) {
 		kfree(buf);