files-reg: make error message in clean_one_remap contain mntns path

rmntns_root - is fd number of opened mntns root directory, these info
doesn't help to identify mntns involved in logs, show path to these
mntns instead.

Signed-off-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@gmail.com>