xfstests, aio-dio-extend-stat.c: fix error reporting

If we receive an unexpected result from an async write, the error
reporting does not tell the actual number of bytes written.  Fix that,
and also a couple of typos in printf's.

Signed-off-by: Jeff Moyer <jmoyer@redhat.com>
Reviewed-by: Rich Johnston <rjohnston@sgi.com>
Signed-off-by: Rich Johnston <rjohnston@sgi.com>