mgr/nfs: fix unintentional recursion

mgr/nfs: fix unintentional recursion

The `exports` property of the ExportMgr exists to cache the exports
configuration found in the .nfs namespace. Using that property
within the property method is probably not intentional and is probably
only working due to the lucky construction of the _exports dict
immediately after the None check so that the _exports dict is returned
(and is a mutable type).

Signed-off-by: John Mulligan <jmulligan@redhat.com>
(cherry picked from commit daa455cd168d62cd8fbcaba4d7aa79b56e68ef0d)