nfsd: pass info about the legacy recoverydir in environment variables

The usermodehelper upcall program can then decide to use this info as
a (one-way) transition mechanism to the new scheme. When a "check"
upcall occurs and the client doesn't exist in the database, we can
look to see whether the directory exists. If it does, then we'd add
the client to the database, remove the legacy recdir, and return
success to the kernel to allow the recovery to proceed.

For gracedone, we simply pass the v4recovery "topdir" so that the
upcall can clean it out prior to returning to the kernel.

A module parm is also added to disable the legacy conversion if
the admin chooses.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>