s3:smbd: use lp_smbd_max_async_dosmode()

s3:smbd: use lp_smbd_max_async_dosmode()

Parametric options have a performance impact, use the normal options
added in the previous commit.

"aio max threads" can only be calculated at run time and requires a
handle to a pthreadpool_tevent which loadparm will never have.

Because of that lp_smbd_max_async_dosmode() will always return 0 as
default and it's up to us to calculate "aio max threads * 2" if
lp_smbd_max_async_dosmode() returns 0.

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>