Validate mountpoint on path-based unmount using statx

Use statx to verify that path-based unmounts proceed only if the
mountpoint reported by statx matches the MNTTAB entry reported by
libzfs, aborting the operation if they differ. Align
`zfs umount /path` behavior with `zfs umount dataset`.

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Ameer Hamza <ahamza@ixsystems.com>
Closes #17481

lib/libspl/include/os/linux/sys/stat.h

@@ -31,6 +31,11 @@
 
 #include <sys/mount.h> /* for BLKGETSIZE64 */
 
+#ifdef HAVE_STATX
+#include <fcntl.h>
+#include <linux/stat.h>
+#endif
+
 /*
  * Emulate Solaris' behavior of returning the block device size in fstat64().
  */