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.

Reviewed by:	cy, imp
Differential Revision:	https://reviews.freebsd.org/D43377