reiserfs: Replace 1-element array with C99 style flex-array

UBSAN found the following issue:

================================================================================
UBSAN: array-index-out-of-bounds in fs/reiserfs/journal.c:4166:22
index 1 is out of range for type '__le32 [1]'

This is because struct reiserfs_journal_desc uses 1-element array for
dynamically sized array member, j_realblock.

This patch fixes this issue by replacing the 1-element array member with C99
style flex-array.  This patch also fixes the same issue in struct
reiserfs_journal_commit as the same manner.

Fixes: f466c6f ("move private bits of reiserfs_fs.h to fs/reiserfs/reiserfs.h")
Signed-off-by: Shigeru Yoshida <syoshida@redhat.com>
Message-Id: <20230821043312.1444068-1-syoshida@redhat.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: Shigeru Yoshida

fs/reiserfs/reiserfs.h

@@ -2699,7 +2699,7 @@ struct reiserfs_iget_args {
 #define get_journal_desc_magic(bh) (bh->b_data + bh->b_size - 12)
 
 #define journal_trans_half(blocksize) \
-	((blocksize - sizeof (struct reiserfs_journal_desc) + sizeof (__u32) - 12) / sizeof (__u32))
+	((blocksize - sizeof(struct reiserfs_journal_desc) - 12) / sizeof(__u32))
 
 /* journal.c see journal.c for all the comments here */
 
@@ -2711,7 +2711,7 @@ struct reiserfs_journal_desc {
 	__le32 j_len;
 
 	__le32 j_mount_id;	/* mount id of this trans */
-	__le32 j_realblock[1];	/* real locations for each block */
+	__le32 j_realblock[];	/* real locations for each block */
 };
 
 #define get_desc_trans_id(d)   le32_to_cpu((d)->j_trans_id)
@@ -2726,7 +2726,7 @@ struct reiserfs_journal_desc {
 struct reiserfs_journal_commit {
 	__le32 j_trans_id;	/* must match j_trans_id from the desc block */
 	__le32 j_len;		/* ditto */
-	__le32 j_realblock[1];	/* real locations for each block */
+	__le32 j_realblock[];	/* real locations for each block */
 };
 
 #define get_commit_trans_id(c) le32_to_cpu((c)->j_trans_id)