aio: avoid request count overflow in ioctx_alloc()

ioctx_alloc() saves the 32-bit nr_events in max_reqs, then doubles
nr_events before checking the ring-size bound.  For
nr_events=0x80000001, the doubled value wraps to 2, so the ring check
passes and aio_setup_ring() builds a one-page/127-slot ring.  The
original 0x80000001 is still stored in ctx->max_reqs and charged to the
global aio_nr quota.

If an administrator has raised fs.aio-max-nr to at least 0x80000001,
an unprivileged process can create that context and make later normal
io_setup() calls fail with -EAGAIN until the wrapped context is
destroyed.

Check for too large number of requests before multiplying.

Link: https://lore.kernel.org/all/20261022091748.3653127-1-teel4res@gmail.com
Reported-by: Cheng Yongkang <teel4res@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://patch.msgid.link/20261022112534.3513745-2-jack@suse.cz
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>