generic/382: Make test independent of fs allocation

generic/382: Make test independent of fs allocation

Test generic/382 depends on filesystem allocating exactly 30M of blocks
when writing 30M file. This is not true for some filesystems - e.g. for
ext2 due to indirect blocks - while leads to false positive failures.
In this case, the test is not actually interested in comparing exact
usage, rather in verifying the ability to write 30M worth of data. So
instead of comparing 'xfs_quota report' output, just depend on detecting
error when writing files.

Signed-off-by: Jan Kara <jack@suse.cz>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>