NFS: Combine nfs_idmap_{init|quit}() and nfs_idmap_{init|quit}_keyring()

The idmap_init() and idmap_quit() functions only exist to call the
_keyring() version.  Let's just call the keyring() functions directly.

Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>