jfs: add check read-only before txBeginAnon() call

Added a read-only check before calling `txBeginAnon` in `extAlloc`
and `extRecord`. This prevents modification attempts on a read-only
mounted filesystem, avoiding potential errors or crashes.

Call trace:
txBeginAnon+0xac/0x154
extAlloc+0xe8/0xdec fs/jfs/jfs_extent.c:78
jfs_get_block+0x340/0xb98 fs/jfs/inode.c:248
__block_write_begin_int+0x580/0x166c fs/buffer.c:2128
__block_write_begin fs/buffer.c:2177 [inline]
block_write_begin+0x98/0x11c fs/buffer.c:2236
jfs_write_begin+0x44/0x88 fs/jfs/inode.c:299

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Reported-by: [e-mail]
Link: https://syzkaller.appspot.com/bug?extid=4e89b5368baba8324e07
Signed-off-by: Vasiliy Kovalev <[e-mail]>
Signed-off-by: Dave Kleikamp <[e-mail]>

fs/jfs/jfs_extent.c

···
 	int rc;
 	int xflag;
 
+	if (isReadOnly(ip)) {
+		jfs_error(ip->i_sb, "read-only filesystem\n");
+		return -EIO;
+	}
+
 	/* This blocks if we are low on resources */
 	txBeginAnon(ip->i_sb);
 
···
 int extRecord(struct inode *ip, xad_t * xp)
 {
 	int rc;
+
+	if (isReadOnly(ip)) {
+		jfs_error(ip->i_sb, "read-only filesystem\n");
+		return -EIO;
+	}
 
 	txBeginAnon(ip->i_sb);