KCSAN find inode->i_disksize could be accessed concurrently.

BUG: KCSAN: data-race in ext4_mark_iloc_dirty / ext4_write_end

write (marked) to 0xffff8b8932f40090 of 8 bytes by task 66792 on cpu 0:
 ext4_write_end+0x53f/0x5b0
 ext4_da_write_end+0x237/0x510
 generic_perform_write+0x1c4/0x2a0
 ext4_buffered_write_iter+0x13a/0x210
 ext4_file_write_iter+0xe2/0x9b0
 new_sync_write+0x29c/0x3a0
 __vfs_write+0x92/0xa0
 vfs_write+0xfc/0x2a0
 ksys_write+0xe8/0x140
 __x64_sys_write+0x4c/0x60
 do_syscall_64+0x8a/0x2a0
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff8b8932f40090 of 8 bytes by task 14414 on cpu 1:
 ext4_mark_iloc_dirty+0x716/0x1190
 ext4_mark_inode_dirty+0xc9/0x360
 ext4_convert_unwritten_extents+0x1bc/0x2a0
 ext4_convert_unwritten_io_end_vec+0xc5/0x150
 ext4_put_io_end+0x82/0x130
 ext4_writepages+0xae7/0x16f0
 do_writepages+0x64/0x120
 __writeback_single_inode+0x7d/0x650
 writeback_sb_inodes+0x3a4/0x860
 __writeback_inodes_wb+0xc4/0x150
 wb_writeback+0x43f/0x510
 wb_workfn+0x3b2/0x8a0
 process_one_work+0x39b/0x7e0
 worker_thread+0x88/0x650
 kthread+0x1d4/0x1f0
 ret_from_fork+0x35/0x40

The plain read is outside of inode->i_data_sem critical section
which results in a data race. Fix it by adding READ_ONCE().
Signed-off-by: Qiujun Huang <hqjagain@gmail.com>
Link: https://lore.kernel.org/r/1582556566-3909-1-git-send-email-hqjagain@gmail.comSigned-off-by: Theodore Ts'o <tytso@mit.edu>