tty: teach the n_tty ICANON case about the new "cookie continuations" too

stable inclusion
from stable-5.10.21
commit c7ff2d25bce3ce820ee537d07f9c73ca1ac00704
bugzilla: 50609

--------------------------------

commit d7fe75cb upstream.

The ICANON case is a bit messy, since it has to look for the line
ending, and has special code to then suppress line ending characters if
they match the __DISABLED_CHAR.  So it actually looks up the line ending
even past the point where it knows it won't copy it to the result
buffer.

That said, apart from all those odd legacy N_TTY ICANON cases, the
actual "should we continue copying" logic isn't really all that
complicated or different from the non-canon case.  In fact, the lack of
"wait for at least N characters" arguably makes the repeat case slightly
simpler.  It really just boils down to "there's more of the line to be
copied".

So add the necessarily trivial logic, and now the N_TTY case will give
long result lines even when in canon mode.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Chen Jun <chenjun102@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>