pgsql: Update parameter name context to wb_context

Update parameter name context to wb_context

For clarity of review, renaming the function parameter "context" in
ScheduleBufferTagForWriteback() and IssuePendingWritebacks() to
"wb_context" is a separate commit. The next commit adds an "io_context"
parameter and "wb_context" makes it more clear which is which.

Author: Melanie Plageman <melanieplageman@gmail.com>
Discussion: https://postgr.es/m/CAAKRu_acc6iL4M3hvOTeztf_ZPpsB3Pqio5aVHgZ5q=Pi3BZKg@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/52676dc2e016df5cd842a4f6073b25ee8e681b9f

Modified Files
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src/backend/storage/buffer/bufmgr.c | 29 +++++++++++++++--------------
src/include/storage/buf_internals.h |  4 ++--
2 files changed, 17 insertions(+), 16 deletions(-)