pgsql: Use the correct sizeof() in BufFileLoadBuffer

Use the correct sizeof() in BufFileLoadBuffer

The sizeof() call should reference buffer.data, because that's the
buffer we're reading data into, not the whole PGAlignedBuffer union.
This was introduced by 44cac93464, which replaced the simple buffer
with a PGAlignedBuffer field.

It's benign, because the buffer is the largest field of the union, so
the sizes are the same. But it's easy to trip over this in a patch, so
fix and backpatch. Commit 44cac93464 went into 12, but that's EOL.

Backpatch-through: 13
Discussion: https://postgr.es/m/928bdab1-6567-449f-98c4-339cd2203b87@vondra.me

Branch
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REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/8cedf6a1b9fcbd33f97413b6a37b38d936522624

Modified Files
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src/backend/storage/file/buffile.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)