MFC r240813:

MFC r240813:
Do not skip two elements of the tid_buffer when reusing the buffer
slot. This eventually results in exhaustion of the tid space, causing
new threads get tid -1 as identifier.

MFC r240951:
Make the updates of the tid ring buffer' head and tail pointers
explicit by moving them into separate statements from the buffer
element accesses.

git-svn-id: svn://svn.freebsd.org/base/stable/9@241071 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f