bufmgr: Pass io_object and io_context through to PinBufferForBlock()

bufmgr: Pass io_object and io_context through to PinBufferForBlock()

PinBufferForBlock() is always_inline and called in a loop in
StartReadBuffersImpl(). Previously it computed io_context and io_object
internally, which required calling IOContextForStrategy() -- a non-inline
function the compiler cannot prove is side-effect-free. This could potential
cause unneeded redundant function calls.

Compute io_context and io_object in the callers instead, allowing
StartReadBuffersImpl() to do so once before entering the loop.

Author: Melanie Plageman <melanieplageman@gmail.com>
Suggested-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/zljergweqti7x67lg5ije2rzjusie37nslsnkjkkby4laqqbfw@3p3zu522yykv