FreeBSD: Remove an incorrect assertion in zfs_getpages()

The pages in the array may become valid after this initial unbusying,
so the assertion only holds during the first iteration of the outer
loop.

Later in zfs_getpages(), the dmu_read_pages() loop handles already-valid
pages. Just drop the assertion, it's not terribly useful.

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Brian Atkinson <batkinson@lanl.gov>
Reported-by: Peter Holm <pho@FreeBSD.org>
Signed-off-by: Mark Johnston <markj@FreeBSD.org>
Sponsored-by: Klara, Inc.
Closes #16810
Closes #16834