stat-util: add helper stat_inode_same() for comparing stat's st_dev/st_ino in one

We do this all over the place, hence let's add a simple helper that does
this and particularly carefully and thoroughly.

src/basic/stat-util.h

@@ -92,6 +92,7 @@ int device_path_parse_major_minor(const char *path, mode_t *ret_mode, dev_t *ret
 
 int proc_mounted(void);
 
+bool stat_inode_same(const struct stat *a, const struct stat *b);
 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);