quota: fix generic/244 on 32-bit platforms

quota: fix generic/244 on 32-bit platforms

The test program src/test-nextquota.c relies on atoi() to convert a
string to an *unsigned* int.  If the string represents an integer
which is greater than INT_MAX, it is undefined how atoi(3) works,
and it turns out that:

       uint id = atoi("2147483649");

results in id == 2147483649 on x86_64, and id == 2147483647 on a
32-bit x86 platform.

So use strtoul(3) instead, which is portable and technically correct

Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>