Fix a signed/unsigned compare warning.
Fix a signed/unsigned compare warning.

Summary:
MSVC2017 with /W4 complains about a signed/unsigned mismatch when destroying
a `ConcurrentHashMap`, due to the `releaseLink` invocation which compares
`count` (an unsigned value) with 1 (signed). Fix this by using `1u` explicitly
instead. Also fix `acquireLink` while I'm at it.

Reviewed By: Orvid

Differential Revision: D41260748

fbshipit-source-id: 0b66bc1ea5622c8841a237cf7ee84a6835a775b4

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly