cephfs: add Unwrap() to error types
cephfs: add Unwrap() to error types

See-also: https://github.com/golang/go/wiki/ErrorValueFAQ#i-have-a-type-that-implements-error-and-holds-a-nested-error-how-should-i-adapt-it-to-the-new-features
Signed-off-by: Niels de Vos <7bda0c0ee7f47171cc808e9e60292cbc5d8140dd@redhat.com>

ceph/ceph-csi,ceph/ceph-csi,ceph/ceph-csi,ceph/ceph-csi