mem: rename lock list to fd list

mem: rename lock list to fd list

Previously, we were only using lock lists to store per-page lock fd's
because we cannot use modern fcntl() file description locks to lock
parts of the page in single file segments mode.

Now, we will be using this list to store either lock fd's (along with
memseg list fd) in single file segments mode, or per-page fd's (and set
memseg list fd to -1), so rename the list accordingly.

Signed-off-by: Anatoly Burakov <anatoly.burakov@intel.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>