hostdisk: fix device detection

Condition was apparently reversed so GRUB assumed all devices were
files. This later made it skip BLKFLSBUF ioctl on Linux which caused
various page cache coherency issues. Observed were

- failure to validate blocklist install (read content did not match
  just written)

- failure to detect Linux MD on disk after online hot addition
  (GRUB got stale superblock)

Closes: 46691

grub-core/kern/emu/hostdisk.c

@@ -161,9 +161,9 @@ grub_util_biosdisk_open (const char *name, grub_disk_t disk)
     {
       struct stat st;
 # if GRUB_DISK_DEVS_ARE_CHAR
-      if (fstat (fd, &st) < 0 || ! S_ISCHR (st.st_mode))
+      if (fstat (fd, &st) >= 0 && S_ISCHR (st.st_mode))
 # else
-      if (fstat (fd, &st) < 0 || ! S_ISBLK (st.st_mode))
+      if (fstat (fd, &st) >= 0 && S_ISBLK (st.st_mode))
 # endif
 	data->is_disk = 1;
     }