io_uring: increase max number of reg buffers

Since recent changes instead of storing a large array of struct
io_mapped_ubuf, we store pointers to them, that is 4 times slimmer and
we should not to so worry about restricting max number of registererd
buffer slots, increase the limit 4 times.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/d3dee1da37f46da416aa96a16bf9e5094e10584d.1620990371.git.asml.silence@gmail.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>