ext4: limit the number of blocks in one ADD_RANGE TLV

Now EXT4_FC_TAG_ADD_RANGE uses ext4_extent to track the
newly-added blocks, but the limit on the max value of
ee_len field is ignored, and it can lead to BUG_ON as
shown below when running command "fallocate -l 128M file"
on a fast_commit-enabled fs:

  kernel BUG at fs/ext4/ext4_extents.h:199!
  invalid opcode: 0000 [#1] SMP PTI
  CPU: 3 PID: 624 Comm: fallocate Not tainted 5.14.0-rc6+ #1
  Hardware name: QEMU Standard PC (i440FX + PIIX, 1996)
  RIP: 0010:ext4_fc_write_inode_data+0x1f3/0x200
  Call Trace:
   ? ext4_fc_write_inode+0xf2/0x150
   ext4_fc_commit+0x93b/0xa00
   ? ext4_fallocate+0x1ad/0x10d0
   ext4_sync_file+0x157/0x340
   ? ext4_sync_file+0x157/0x340
   vfs_fsync_range+0x49/0x80
   do_fsync+0x3d/0x70
   __x64_sys_fsync+0x14/0x20
   do_syscall_64+0x3b/0xc0
   entry_SYSCALL_64_after_hwframe+0x44/0xae

Simply fixing it by limiting the number of blocks
in one EXT4_FC_TAG_ADD_RANGE TLV.

Fixes: aa75f4d3 ("ext4: main fast-commit commit path")
Cc: stable@kernel.org
Signed-off-by: Hou Tao <houtao1@huawei.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Link: https://lore.kernel.org/r/20210820044505.474318-1-houtao1@huawei.com

fs/ext4/fast_commit.c

@@ -892,6 +892,12 @@ static int ext4_fc_write_inode_data(struct inode *inode, u32 *crc)
 					    sizeof(lrange), (u8 *)&lrange, crc))
 				return -ENOSPC;
 		} else {
+			unsigned int max = (map.m_flags & EXT4_MAP_UNWRITTEN) ?
+				EXT_UNWRITTEN_MAX_LEN : EXT_INIT_MAX_LEN;
+
+			/* Limit the number of blocks in one extent */
+			map.m_len = min(max, map.m_len);
+
 			fc_ext.fc_ino = cpu_to_le32(inode->i_ino);
 			ex = (struct ext4_extent *)&fc_ext.fc_ex;
 			ex->ee_block = cpu_to_le32(map.m_lblk);