f2fs: flag as supporting buffered async reads

The f2fs uses generic_file_buffered_read(), which supports buffered async
reads since commit 1a0a7853 ("mm: support async buffered reads in
generic_file_buffered_read()").

Let's enable it to match other file-systems. The read performance has been
greatly improved under io_uring:

    167M/s -> 234M/s, Increase ratio by 40%

Test w/:
    ./fio --name=onessd --filename=/data/test/local/io_uring_test
    --size=256M --rw=randread --bs=4k --direct=0 --overwrite=0
    --numjobs=1 --iodepth=1 --time_based=0 --runtime=10
    --ioengine=io_uring --registerfiles --fixedbufs
    --gtod_reduce=1 --group_reporting --sqthread_poll=1

Signed-off-by: Lu Hongfei <luhongfei@vivo.com>
Signed-off-by: Yangtao Li <frank.li@vivo.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>