ext4: submit and wait for disksize-grow I/O in fallocate paths

Collapse range and insert range update i_disksize directly under
i_data_sem. If the operation runs while the zeroed EOF block is still
awaiting writeback, i_disksize could advance past the zeroed boundary
before the zeroed data is persisted, exposing stale data on crash.

Deferring i_disksize updates like fallocate and zero_range is not an
option here because the shift would move written extents beyond the
current i_disksize. So flush and wait for the pending zeroed EOF block
before these operations advance i_disksize. Since these operations
already perform writeback, the extra flush does not add significant
overhead.

In addition, for ext4_update_disksize_before_punch(), if the punch
discards the pending block, the zeroed data will never be written back
before advancing i_disksize, so it is also necessary to sync the
pending EOF range there.

Finally, for the SYNC variants of zero_range and fallocate, this also
guarantees the i_disksize update is persisted on the synchronous
return.

Note that EXT4_STATE_DISKSIZE_GROW_PENDING is not set for now, and it
will be set after everything is done.

Signed-off-by: Zhang Yi <yi.zhang@huawei.com>
Tested-by: syzbot@syzkaller.appspotmail.com
Link: https://patch.msgid.link/20261009103033.2920530-26-yi.zhang@huaweicloud.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

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