files: change error to warning in linkat_hard error path

We print errors in all error cases when calling linkat_hard anyway, but
for some errors like EEXIST we are fine and just skip them, so we should
not print error here.

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