Fix some nits in zfs_getpages()

• If we don't want dmu_read_pages() to perform extra readahead/behind, pass a pointer to 0 instead of a null pointer, as dum_read_pages() expects rahead and rbehind to be non-null.
• Avoid unneeded iterations in a loop.

Sponsored-by: Klara, Inc.
Reported-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Brian Atkinson <batkinson@lanl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Mark Johnston <markj@FreeBSD.org>
Closes #16758