fs/ext4-new-features: Add absolute path for the test

After commit 33a4adff42a0f4f8310774bb689f42912505b51c
tests need absolute path. If not given it would return TBROK.
Add an absolute path before running fallocate0${i} in ext4-new-features.

Signed-off-by: Zeng Linggang <zenglg.jy@cn.fujitsu.com>
Acked-by: Cyril Hrubis <chrubis@suse.cz>