fsck_msdosfs: fix FAT header correction not persisting in cache mode

When fsck_msdosfs runs with FAT32 cache mode (used for large
filesystems that cannot be mmap'd), a detected FAT header correction
was written into the in-memory buffer but the corresponding cache
entry (fat32_cache_allentries[0]) was never marked dirty.  As a
result, fat_flush_fat32_cache_entry() skipped it, the corrected
bytes were never written to disk, and copyfat() propagated the
uncorrected on-disk data to all backup FAT copies.  Every subsequent
fsck run would repeat the same "FAT starts with odd byte sequence /
FIXED" cycle indefinitely.

Fix by marking fat32_cache_allentries[0].dirty = true after applying
the in-memory correction, ensuring the chunk is flushed before
copyfat() runs.

Obtained from:	https://android-review.googlesource.com/c/platform/external/fsck_msdos/+/4047981
MFC after:	3 days

sbin/fsck_msdosfs/fat.c

@@ -927,6 +927,17 @@ readfat(int fs, struct bootblock *boot, struct fat_descriptor **fp)
 				default:
 					break;
 				}
+				/*
+				 * In cache mode the header lives in
+				 * fat32_cache_allentries[0].  Mark it
+				 * dirty so it is flushed to disk (either
+				 * on eviction or in writefat()) before
+				 * copyfat() copies the primary FAT to
+				 * backup copies.
+				 */
+				if (fat->use_cache)
+					fat->fat32_cache_allentries[0].dirty =
+					    true;
 			}
 		}
 	}