/lib/x86_64-linux-gnu/libmount.so.1 is in libmount1 2.29.2-1+deb9u1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /lib/x86_64-linux-gnu/libmount.so.1.1.0
/lib/x86_64-linux-gnu/libmount.so.1.1.0