libceph, ceph: avoid memory leak when specifying same option several times

[ Upstream commit 937441f3a3158d5510ca8cc78a82453f57a96365 ]

When parsing string option, in order to avoid memory leak we need to
carefully free it first in case of specifying same option several times.

Signed-off-by: Chengguang Xu <cgxu519@icloud.com>
Reviewed-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>