fs/ext4-new-features: fix wrong test point when running ext4-persist-prealloc

ext4-persist-prealloc test will call test cases under 'testcases/kernel/syscalls
/fallocate' to have persist prealloc test for ext4 filesystem. But currently
fallocate[0-3] cases will have tests in a different filesystem, which the directory
created by tst_tmpdir() is in. This is wrong, fix it.

Signed-off-by: Xiaoguang Wang <wangxg.fnst@cn.fujitsu.com>