NFS: exit_nfs_v4() shouldn't be an __exit function

... yet.  Right now, init_nfs() is calling this function if an error is
encountered when loading the nfs module.  An __exit function can't be
called from one declared as __init.

Signed-off-by: Bryan Schumaker <bjschuma@netapp.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>