tree-wide: use read_full_virtual_file() where appropriate

Wherever we read virtual files we better should use
read_full_virtual_file(), to make sure we get a consistent response
given how weird the kernel's handling with partial read on such file
systems is.

src/basic/mountpoint-util.c

@@ -114,7 +114,7 @@ static int fd_fdinfo_mnt_id(int fd, const char *filename, int flags, int *ret_mn
                 xsprintf(path, "/proc/self/fdinfo/%i", subfd);
         }
 
-        r = read_full_file(path, &fdinfo, NULL);
+        r = read_full_virtual_file(path, &fdinfo, NULL);
         if (r == -ENOENT) /* The fdinfo directory is a relatively new addition */
                 return -EOPNOTSUPP;
         if (r < 0)