smgrextendv and vectorizing the bulk_write implementation

Hi,

While working on the fix for [0] I noticed that bulk_write doens't use
any of the new vectorized IO features, which seemed like a waste.
After looking into it a bit deeper, I noticed the opportunity for
write vectorization was not very high, as one would expect most
bulk_write IOs to be smgrextend(), which only does page-sized writes.
That's something that can be solved, though, and so I started this
patch.

I've attached two patches to address these two items:

Patch 1/2 reworks smgrextend to smgrextendv, which does mostly the
same stuff as the current smgrextend, but operates on multiple pages.
Patch 2/2 updates bulk_write to make use of smgrwritev,
smgrzeroextend, and the new smgrextendv API, thus reducing the syscall
burden in processes that use bulk extend APIs.

Open question:
In my version of smgrextendv, I reject any failure to extend by the
requested size. This is different from smgrwrite, which tries to write
again when FileWriteV returns a short write. Should smgrextendv do
retries, too?

Kind regards,

Matthias van de Meent
Neon (https://neon.tech)

[0] https://postgr.es/m/flat/CACAa4VJ%2BQY4pY7M0ECq29uGkrOygikYtao1UG9yCDFosxaps9g%40mail.gmail.com