udf: Fix crash on IO error during truncate

When truncate(2) hits IO error when reading indirect extent block the
code just bugs with:

kernel BUG at linux-4.15.0/fs/udf/truncate.c:249!
...

Fix the problem by bailing out cleanly in case of IO error.

CC: stable@vger.kernel.org
Reported-by: jean-luc malet <jeanluc.malet@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>

fs/udf/truncate.c

@@ -260,6 +260,9 @@ void udf_truncate_extents(struct inode *inode)
 			epos.block = eloc;
 			epos.bh = udf_tread(sb,
 					udf_get_lb_pblock(sb, &eloc, 0));
+			/* Error reading indirect block? */
+			if (!epos.bh)
+				return;
 			if (elen)
 				indirect_ext_len =
 					(elen + sb->s_blocksize - 1) >>