btrfs-progs: tests: fix fsck/024 to work with free-space-tree

The test looks for a pattern FREE_SPACE that also matches
FREE_SPACE_TREE when it's enabled. Use word regexp to match the whole
word only.

Signed-off-by: David Sterba <dsterba@suse.com>

tests/fsck-tests/024-clear-space-cache/test.sh

@@ -36,7 +36,7 @@ run_check "$TOP/btrfs" check "$TEST_DEV"
 
 # Manually recheck space cache and super space cache generation
 run_check_stdout "$TOP/btrfs" inspect-internal dump-tree -t root "$TEST_DEV" | \
-	grep -q FREE_SPACE
+	grep -q -w FREE_SPACE
 if [ $? -eq 0 ]; then
 	_fail "clear space cache doesn't clear all space cache"
 fi