stat-util: provide single fallback implementation of statx()

This simplifies things quite a bit, and is reusable wherever we want to
use statx() later on. Not sure why I didn't do it like this right from
the beginning...

src/basic/stat-util.h

@@ -10,6 +10,7 @@
 #include <sys/vfs.h>
 
 #include "macro.h"
+#include "missing_stat.h"
 
 int is_symlink(const char *path);
 int is_dir(const char *path, bool follow);
@@ -92,6 +93,8 @@ int proc_mounted(void);
 
 bool stat_inode_unmodified(const struct stat *a, const struct stat *b);
 
+int statx_fallback(int dfd, const char *path, int flags, unsigned mask, struct statx *sx);
+
 #if HAS_FEATURE_MEMORY_SANITIZER
 #  warning "Explicitly initializing struct statx, to work around msan limitation. Please remove as soon as msan has been updated to not require this."
 #  define STRUCT_STATX_DEFINE(var)              \