Replace the BUG_ON(idmap->idmap_key_cons != NULL) with a
WARN_ON_ONCE(). Then get rid of the ACCESS_ONCE(idmap->idmap_key_cons).

Then add helper functions for starting, finishing and aborting the
legacy upcall.
Signed-off-by: NTrond Myklebust <Trond.Myklebust@netapp.com>
Cc: Bryan Schumaker <bjschuma@netapp.com>