f2fs: fix avoid race between truncate and background GC

Thread A				Background GC
- f2fs_setattr isize to 0
 - truncate_setsize
					- gc_data_segment
					 - f2fs_get_read_data_page page #0
					  - set_page_dirty
					  - set_cold_data
 - f2fs_truncate

- f2fs_setattr isize to 4k
- read 4k <--- hit data in cached page #0

Above race condition can cause read out invalid data in a truncated
page, fix it by i_gc_rwsem[WRITE] lock.

Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>