linkat/linkat02.c: fix xfs with EMLINK issue

As Shuang Qiu <shuang.qiu@oracle.com> reports:
* The 32 bit link count in the inode theoretically maxes out at UINT_MAX.
* Since the pathconf interface is signed, we use 2^31 - 1 instead.
* The old inode format had a 16 bit link count, so its maximum is USHRT_MAX.
We are not able to reach such limitation.

If the mntpoint is xfs, we skip the setup for EMLINK and return TCONF in the
EMLINK test.

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