mountpoint-util: use new kernel 5.8 statx() API for determining mnt_id

The kernel finally has a proper API to determine the mnt_id of a file.
Let's use it.

This adds support for the STATX_MNT_ID field of statx(), added in
kernel 5.8.

src/basic/missing_syscall.h

@@ -482,7 +482,7 @@ static inline ssize_t missing_statx(int dfd, const char *filename, unsigned flag
 #  endif
 }
 
-#  define statx missing_statx
+#  define statx(dfd, filename, flags, mask, buffer) missing_statx(dfd, filename, flags, mask, buffer)
 #endif
 
 #if !HAVE_SET_MEMPOLICY