configure: Add mkdirat() detection, fix build

Build failure is caused by adding the ex4 features tests into the
default build which exposes code which uses mkdirat().

This patch adds configure check for mkdirat() and ifdefs around the
code in create_long_dirs.c and create_short_dirs.c.

Signed-off-by: Cyril Hrubis <chrubis@suse.cz>
diff --git a/configure.ac b/configure.ac
index 9e3df00..8a2a96a 100644
--- a/configure.ac
+++ b/configure.ac
@@ -172,5 +172,6 @@
 LTP_CHECK_KERNEL_DEVEL
 LTP_CHECK_XFS_QUOTACTL
 LTP_CHECK_CLONE_SUPPORTS_7_ARGS
+LTP_CHECK_MKDIRAT
 
 AC_OUTPUT