Provide multi-block smgrprefetch().

Provide multi-block smgrprefetch().

Previously smgrprefetch() could issue POSIX_FADV_WILLNEED advice for a
single block at a time.  Add an nblocks argument so that we can do the
same for a range of blocks.  This usually produces a single system call,
but might need to loop if it crosses a segment boundary.  Initially it
is only called with nblocks == 1, but proposed patches will make wider
calls.

Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi> (earlier version)
Discussion: https://postgr.es/m/CA+hUKGJkOiOCa+mag4BF+zHo7qo=o9CFheB8=g6uT5TUm2gkvA@mail.gmail.com