cifs: fix signed/unsigned mismatch on aio_read patch

The patch "CIFS: Add support for direct I/O read" had
a signed/unsigned mismatch (ssize_t vs. size_t) in the
return from one function.  Similar trivial change
in aio_write

Signed-off-by: Long Li <longli@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>
Reported-by: Julia Lawall <julia.lawall@lip6.fr>