Don't return ELOOP when crossing a mountpoint boundary from a BK volume to

Don't return ELOOP when crossing a mountpoint boundary from a BK volume to
a BK volume.  find treats it as a fatal error, and it's annoying not to be
able to naively use find to recurse through a volume with its backup volume
mounted.  Never noticed this before because we used to ignore this return
value.  Return ENODEV instead.

(cherry picked from commit f6462eac361ba2bddf5b84cdb7861f7ca482caba)