testcases/kernel/fs/doio/doio.c: Fix buffer size.

Use 'strlen(exec_path) + 2' instead of 'strlen(exec_path + 1)'

Signed-off-by: Cyril Hrubis <chrubis@suse.cz>
diff --git a/testcases/kernel/fs/doio/doio.c b/testcases/kernel/fs/doio/doio.c
index 656642b..dd4a50b 100644
--- a/testcases/kernel/fs/doio/doio.c
+++ b/testcases/kernel/fs/doio/doio.c
@@ -650,9 +650,7 @@
 					char *exec_path;
 
 					exec_path = argv[0];
-					argv[0] =
-					    (char *)
-					    malloc(strlen(exec_path + 1));
+					argv[0] = malloc(strlen(exec_path) + 2);
 					sprintf(argv[0], "-%s", exec_path);
 
 					execvp(exec_path, argv);