selftests/mm: compaction_test: fix off by one in check_compaction()

The "initial_nr_hugepages" variable is unsigned long so it takes up to 20
characters to print, plus 1 more character for the NUL terminator. 
Unfortunately, this buffer is not quite large enough for the terminator to
fit.  Also use snprintf() for a belt and suspenders approach.

Link: https://lkml.kernel.org/r/87470c06-b45a-4e83-92ff-aac2e7b9c6ba@stanley.mountain
Fixes: fb9293b ("selftests/mm: compaction_test: fix bogus test success and reduce probability of OOM-killer invocation")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: Dan Carpenter

tools/testing/selftests/mm/compaction_test.c

@@ -89,9 +89,10 @@ int check_compaction(unsigned long mem_free, unsigned long hugepage_size,
 	int fd, ret = -1;
 	int compaction_index = 0;
 	char nr_hugepages[20] = {0};
-	char init_nr_hugepages[20] = {0};
+	char init_nr_hugepages[24] = {0};
 
-	sprintf(init_nr_hugepages, "%lu", initial_nr_hugepages);
+	snprintf(init_nr_hugepages, sizeof(init_nr_hugepages),
+		 "%lu", initial_nr_hugepages);
 
 	/* We want to test with 80% of available memory. Else, OOM killer comes
 	   in to play */