ceph-volume: fix raw activate when device path is stale

ceph-volume: fix raw activate when device path is stale

This changes unlink_bs_symlinks to use os.path.lexists instead
of os.path.exists. It can happen that devices get renumbered,
in that case, the OSD symlink still exists but its target device
is gone which means os.path.exists returns False, so the symlink
is never cleaned up and ceph-volume activate can fail later.

Fixes: https://tracker.ceph.com/issues/77295
Signed-off-by: Guillaume Abrioux <gabrioux@ibm.com>
(cherry picked from commit 1ee1fc14fe16aee940293ec228107e8c5a68c2d8)