PG17beta2: SMGR: inconsistent type for nblocks

Hi,

While working on rebasing the patches of Neon's fork onto the
REL_17_STABLE branch, I noticed that the nblocks arguments of various
smgr functions have inconsistent types: smgrzeroextend accepts
`nblocks` as signed integer, as does the new signature for
smgrprefetch, but the new vectorized operations of *readv and *writev,
and the older *writeback all use an unsigned BlockNumber as indicator
for number of blocks.

Can we update the definition to be consistent across this (new, or
also older) API? As far as I can see, in none of these cases are
negative numbers allowed or expected, so updating this all to be
consistently BlockNumber across the API seems like a straigthforward
patch.

cc-ed Thomas as committer of the PG17 smgr API changes.

Kind regards,

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