lib/tst_fs_has_free.c: add tst_fs_has_free() to determine filesystem's free space

The existing lib/tst_cwd_has_free.c only determines the filesystem, which
the current directory is in. And I think tst_cwd_has_free() is not entirely
correct. See this code in tst_cwd_has_free():
	return ((float)sf.f_bfree) / (1024 / sf.f_bsize) >=
		 required_kib ? 1 : 0;
if sf.f_bsize is greater than 1024, this code is wrong.

I choose to remove tst_cwd_has_free.c and add lib/tst_fs_has_free.c.
The prototype is below:
	int tst_fs_has_free(void (*cleanup)(void), const char *path,
		unsigned int size, unsigned int mult);

User can specify the path to determine the corresponding filesystem.

Signed-off-by: Xiaoguang Wang <wangxg.fnst@cn.fujitsu.com>
Signed-off-by: Cyril Hrubis <chrubis@suse.cz>