io_uring: ensure openat sets O_LARGEFILE if needed

OPENAT2 correctly sets O_LARGEFILE if it has to, but that escaped the
OPENAT opcode. Dmitry reports that his test case that compares openat()
and IORING_OP_OPENAT sees failures on large files:

*** sync openat
openat succeeded
sync write at offset 0
write succeeded
sync write at offset 4294967296
write succeeded

*** sync openat
openat succeeded
io_uring write at offset 0
write succeeded
io_uring write at offset 4294967296
write succeeded

*** io_uring openat
openat succeeded
sync write at offset 0
write succeeded
sync write at offset 4294967296
write failed: File too large

*** io_uring openat
openat succeeded
io_uring write at offset 0
write succeeded
io_uring write at offset 4294967296
write failed: File too large

Ensure we set O_LARGEFILE, if force_o_largefile() is true.

Cc: stable@vger.kernel.org # v5.6
Fixes: 15b71abe ("io_uring: add support for IORING_OP_OPENAT")
Reported-by: Dmitry Kadashev <dkadashev@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>