kern: tty: fix ttyinq_read_uio assertion

kern: tty: fix ttyinq_read_uio assertion

It's clear from later context that `rlen` was always expected to include
`flen`, as we'll trim `flen` bytes from the end of the read.  Relax our
initial assertion to only require the total size less trimmed bytes to
lie within the out buffer size.

While we're here, I note that if we have to read more than one block and
we're trimming from the end then we'll do the wrong thing and omit
`flen` bytes from every block, rather than just the end.  Add an
assertion to make sure we're not doing that, but the only caller that
specifies a non-zero `flen` today will only really be doing so if rlen
is entirely within a single buffer.

Approved by:	re (cperciva)
Reviewed by:	cy, imp

(cherry picked from commit 09a43b8790bdeb97fbecd3ea767c2f599eb4a4d3)
(cherry picked from commit 81ef0de636ff8ba0b8057ced593f2ab92597b1a6)