reftable/stack: do not die when fsyncing lock file files

reftable/stack: do not die when fsyncing lock file files

We use `fsync_component_or_die()` when committing an addition to the
"tables.list" lock file, which unsurprisingly dies in case the fsync
fails. Given that this is part of the reftable library, we should never
die and instead let callers handle the error.

Adapt accordingly and use `fsync_component()` instead.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>