qa: fixup quota values exceeding 64 bit signed int value

qa: fixup quota values exceeding 64 bit signed int value

The test does the following:

        setfattr -n ceph.quota.max_bytes -v 8388608Ti .

This failes since 8388608Ti exceeds 64 bit signed integer value.
Change that to the approprivate value and expect faliure when
quota is set for this exceeded value.

Fixes: http://tracker.ceph.com/issues/55940
Introduced-by: 733065148250e892fca941cdeaad3e3a8193b0d8
Signed-off-by: Venky Shankar <vshankar@redhat.com>