Revert "fstests: Don't use gawk's strtonum"

This reverts commit 37520a314b.

This commit has been reported to regress, at least, xfs/139 and
btrfs/09[58]. Let's revert it for now.

Signed-off-by: Eryu Guan <guaneryu@gmail.com>

tests/generic/386

@@ -75,7 +75,7 @@ _filter_quota_rpt() {
 	# This function parses the human-readable values produced
 	# by xfs_quota output
 	function byte_size(value,  result) {
-		result = value;
+		result = strtonum(value);
 		unit = value;
 		gsub("[0-9][0-9]*", "", unit);
 		shift = index("KMGTPE", unit);