libmount: use AT_STATX_DONT_SYNC when touch mountpoints

libmount: use AT_STATX_DONT_SYNC when touch mountpoints

* prefer statx() with AT_STATX_DONT_SYNC if available
* keep fstatat() and stat() as fallback
* add test to mnt_stat_mountpoint()

The goal is to minimize situations when we need classic stat() because
it triggers automount, and stat() syscall may hang on unreachable
network filesystems. The automount issue was resolved years ago by
AT_NO_AUTOMOUNT; now we can use statx() to fix also hangs on NFS.

Addresses: https://github.com/util-linux/util-linux/issues/2049
Signed-off-by: Karel Zak <kzak@redhat.com>