f2fs: fix to truncate preallocated blocks in f2fs_file_open()

[ Upstream commit 298b1e41 ]

chenyuwen reports a f2fs bug as below:

Unable to handle kernel NULL pointer dereference at virtual address 0000000000000011
 fscrypt_set_bio_crypt_ctx+0x78/0x1e8
 f2fs_grab_read_bio+0x78/0x208
 f2fs_submit_page_read+0x44/0x154
 f2fs_get_read_data_page+0x288/0x5f4
 f2fs_get_lock_data_page+0x60/0x190
 truncate_partial_data_page+0x108/0x4fc
 f2fs_do_truncate_blocks+0x344/0x5f0
 f2fs_truncate_blocks+0x6c/0x134
 f2fs_truncate+0xd8/0x200
 f2fs_iget+0x20c/0x5ac
 do_garbage_collect+0x5d0/0xf6c
 f2fs_gc+0x22c/0x6a4
 f2fs_disable_checkpoint+0xc8/0x310
 f2fs_fill_super+0x14bc/0x1764
 mount_bdev+0x1b4/0x21c
 f2fs_mount+0x20/0x30
 legacy_get_tree+0x50/0xbc
 vfs_get_tree+0x5c/0x1b0
 do_new_mount+0x298/0x4cc
 path_mount+0x33c/0x5fc
 __arm64_sys_mount+0xcc/0x15c
 invoke_syscall+0x60/0x150
 el0_svc_common+0xb8/0xf8
 do_el0_svc+0x28/0xa0
 el0_svc+0x24/0x84
 el0t_64_sync_handler+0x88/0xec

It is because inode.i_crypt_info is not initialized during below path:
- mount
 - f2fs_fill_super
  - f2fs_disable_checkpoint
   - f2fs_gc
    - f2fs_iget
     - f2fs_truncate

So, let's relocate truncation of preallocated blocks to f2fs_file_open(),
after fscrypt_file_open().

Fixes: d4dd19ec

 ("f2fs: do not expose unwritten blocks to user by DIO")
Reported-by: chenyuwen <yuwen.chen@xjmz.com>
Closes: https://lore.kernel.org/linux-kernel/20240517085327.1188515-1-yuwen.chen@xjmz.com
Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>