ext4: defer i_disksize update while DISKSIZE_GROW_PENDING is set

Operations like append allocate, zero range, and truncate update
i_disksize directly. If the new i_disksize exceeds the original value
while the zeroed EOF block is still awaiting writeback, metadata may be
persisted before the zeroed data, exposing stale data on crash.

Defer i_disksize updates while EXT4_STATE_DISKSIZE_GROW_PENDING is set;
the ioend worker for the pending block will advance i_disksize to i_size
once the zeroed data is written back. The tradeoff is that i_disksize
may lag i_size transiently, but this is observable only to callers that
read i_disksize directly.

Introduce __ext4_set_i_disksize() to centralize the bit check for
callers already holding i_data_sem (ext4_ext_truncate and
ext4_set_inode_size), and refactor ext4_update_inode_size() to take
i_data_sem itself and check the bit atomically with i_size_write(), so
the ioend worker observes the latest i_size under the same lock.

Note that in the O_SYNC mode of ext4_do_fallocate() or
ext4_zero_range(), we need to flush out pending blocks that can update
i_disksize synchronously. This will be handled later, and
EXT4_STATE_DISKSIZE_GROW_PENDING will also be set after everything is
done.

Suggested-by: Jan Kara <jack@suse.cz>
Signed-off-by: Zhang Yi <yi.zhang@huawei.com>
Link: https://patch.msgid.link/20260903123543.2302999-25-yi.zhang@huaweicloud.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

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