ovl: implement async IO routines

A performance regression was observed since linux v4.19 with aio test using
fio with iodepth 128 on overlayfs.  The queue depth of the device was
always 1 which is unexpected.

After investigation, it was found that commit 16914e6f ("ovl: add
ovl_read_iter()") and commit 2a92e07e ("ovl: add ovl_write_iter()")
resulted in vfs_iter_{read,write} being called on underlying filesystem,
which always results in syncronous IO.

Implement async IO for stacked reading and writing.  This resolves the
performance regresion.

This is implemented by allocating a new kiocb for submitting the AIO
request on the underlying filesystem.  When the request is completed, the
new kiocb is freed and the completion callback is called on the original
iocb.

Signed-off-by: Jiufei Xue <jiufei.xue@linux.alibaba.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>