lib: add tst_fs_link_count.c

Add two test interfaces:

int tst_fs_fill_hardlinks(void (*cleanup) (void), const char *dir);

We can use tst_fs_fill_hardlinks() to get maximum number of hard links to
regular file in a mounted file system, which @dir is in.

int tst_fs_fill_dir(void (*cleanup) (void), const char *dir);

We can use tst_fs_fill_dir() to get maximum number of elements under
directory @dir in a mounted file system.

Signed-off-by: Xiaoguang Wang <wangxg.fnst@cn.fujitsu.com>
Signed-off-by: Cyril Hrubis <chrubis@suse.cz>
linkat/linkat02.c: Fix EMLINK case

Remove the check against XFS when testing EMLINK error value.
Using the new interface provided by lib/tst_fs_link_count.c.

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