udf: Prevent write-unsupported filesystem to be remounted read-write

There are certain filesystem features which we support for reading but
not for writing. We properly refuse to mount such filesystems read-write
however for some features (such as read-only partitions), we don't check
for these features when remounting the filesystem from read-only to
read-write. Thus such filesystems could be remounted read-write leading
to strange behavior (most likely crashes).

Fix the problem by marking in superblock whether the filesystem has some
features that are supported in read-only mode and check this flag during
remount.

Signed-off-by: Jan Kara <jack@suse.cz>

fs/udf/udf_sb.h

@@ -30,6 +30,8 @@
 #define UDF_FLAG_LASTBLOCK_SET	16
 #define UDF_FLAG_BLOCKSIZE_SET	17
 #define UDF_FLAG_INCONSISTENT	18
+#define UDF_FLAG_RW_INCOMPAT	19	/* Set when we find RW incompatible
+					 * feature */
 
 #define UDF_PART_FLAG_UNALLOC_BITMAP	0x0001
 #define UDF_PART_FLAG_UNALLOC_TABLE	0x0002