bufmgr: use I/O stats arguments in FlushUnlockedBuffer()

bufmgr: use I/O stats arguments in FlushUnlockedBuffer()

FlushUnlockedBuffer() accepted io_object and io_context arguments but
hardcoded IOOBJECT_RELATION and IOCONTEXT_NORMAL when calling
FlushBuffer(). Pass them through instead. Also fix FlushBuffer() to use
its io_object parameter for I/O timing stats rather than hardcoding
IOOBJECT_RELATION.

Not actively broken since all current callers pass IOOBJECT_RELATION and
IOCONTEXT_NORMAL, so not backpatched.

Author: Chao Li <lic@highgo.com>
Reviewed-by: Melanie Plageman <melanieplageman@gmail.com>
Discussion: https://postgr.es/m/BC97546F-5C15-42F2-AD57-CFACDB9657D0@gmail.com