f2fs: cover last_disk_size update with spinlock

f2fs: cover last_disk_size update with spinlock

This change solves below hangtask issue:

INFO: task kworker/u16:1:58 blocked for more than 122 seconds.
      Not tainted 5.6.0-rc2-00590-g9983bdae4974e #11
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
kworker/u16:1   D    0    58      2 0x00000000
Workqueue: writeback wb_workfn (flush-179:0)
Backtrace:
 (__schedule) from [<c0913234>] (schedule+0x78/0xf4)
 (schedule) from [<c017ec74>] (rwsem_down_write_slowpath+0x24c/0x4c0)
 (rwsem_down_write_slowpath) from [<c0915f2c>] (down_write+0x6c/0x70)
 (down_write) from [<c0435b80>] (f2fs_write_single_data_page+0x608/0x7ac)
 (f2fs_write_single_data_page) from [<c0435fd8>] (f2fs_write_cache_pages+0x2b4/0x7c4)
 (f2fs_write_cache_pages) from [<c043682c>] (f2fs_write_data_pages+0x344/0x35c)
 (f2fs_write_data_pages) from [<c0267ee8>] (do_writepages+0x3c/0xd4)
 (do_writepages) from [<c0310cbc>] (__writeback_single_inode+0x44/0x454)
 (__writeback_single_inode) from [<c03112d0>] (writeback_sb_inodes+0x204/0x4b0)
 (writeback_sb_inodes) from [<c03115cc>] (__writeback_inodes_wb+0x50/0xe4)
 (__writeback_inodes_wb) from [<c03118f4>] (wb_writeback+0x294/0x338)
 (wb_writeback) from [<c0312dac>] (wb_workfn+0x35c/0x54c)
 (wb_workfn) from [<c014f2b8>] (process_one_work+0x214/0x544)
 (process_one_work) from [<c014f634>] (worker_thread+0x4c/0x574)
 (worker_thread) from [<c01564fc>] (kthread+0x144/0x170)
 (kthread) from [<c01010e8>] (ret_from_fork+0x14/0x2c)

Reported-and-tested-by: Ondřej Jirman <megi@xff.cz>
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>