NFSv4: Ensure that nfs4_alloc_client cleans up on error.

Any pointer that was allocated through nfs_alloc_client() needs to be
freed via a call to nfs_free_client().

Reported-by: Stanislav Kinsbursky <skinsbursky@parallels.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>