zfs: fix assert in FreeBSD-specific dmu_read_pages

zfs: fix assert in FreeBSD-specific dmu_read_pages

From the openzfs 2e160dee9 commit message:
  The function has three similar pieces of code: for read-behind pages,
  requested pages and read-ahead pages.  All three pieces had an
  assert to ensure that the page is not mapped.  Later the assert was
  relaxed to require that the page is not mapped for writing.  But that
  was done in two places out of three.  This change fixes the third piece,
  read-ahead.

Obtained from:	openzfs/zfs@2e160dee97a4badbc318561f76ea56451c916d2f
MFS after:	3 days

(direct commit)