pgsql: bufmgr: Restructure AsyncReadBuffers()

bufmgr: Restructure AsyncReadBuffers()

Restructure AsyncReadBuffers() to use early return when the head buffer is
already valid, instead of using a did_start_io flag and if/else branches. Also
move around a bit of the code to be located closer to where it is used. This
is a refactor only.

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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/8a1a1d6ab86217020a612ee2e03eae4fe00ae781

Modified Files
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src/backend/storage/buffer/bufmgr.c | 157 +++++++++++++++++++-----------------
1 file changed, 81 insertions(+), 76 deletions(-)