bufmgr: Fix valgrind checking for buffers pinned in StrategyGetBuffer()

bufmgr: Fix valgrind checking for buffers pinned in StrategyGetBuffer()

In 5e899859287 I made StrategyGetBuffer() pin buffers with a single CAS,
instead of using PinBuffer_Locked(). Unfortunately I missed that
PinBuffer_Locked() marked the page as defined for valgrind.

Fix this oversight by centralizing the valgrind initialization into
TrackNewBufferPin(), which also allows us to reduce the number of places doing
VALGRIND_MAKE_MEM_DEFINED.

Per buildfarm animal skink and Amit Langote.

Discussion: https://postgr.es/m/fvfmkr5kk4nyex56ejgxj3uzi63isfxovp2biecb4bspbjrze7@az2pljabhnff
Discussion: https://postgr.es/m/CA+HiwqGKJ6nEXEPQW7EpykVsEtzxp5-up_xhtcUAkWFtATVQvQ@mail.gmail.com