Available AIO contexts checks wraps around when aio-nr > aio-max-nr

[fee-mendes]

It turns out that an user can rightfully have a aio-nr value bigger than aio-max-nr. Such situation happens when aio-max-nr value is modified on the fly and the running system has active AIO contexts, for example:

$ sudo sysctl -w fs.aio-max-nr=0
fs.aio-max-nr = 0
$ sudo sysctl -a | grep aio
fs.aio-max-nr = 0
fs.aio-nr = 102052

Therefore, the corresponding checks must be signed integers. Found after work under #1058