ext4: properly sync file size update after O_SYNC direct IO

Gao Xiang has reported that on ext4 O_SYNC direct IO does not properly
sync file size update and thus if we crash at unfortunate moment, the
file can have smaller size although O_SYNC IO has reported successful
completion. The problem happens because update of on-disk inode size is
handled in ext4_dio_write_iter() *after* iomap_dio_rw() (and thus
dio_complete() in particular) has returned and generic_file_sync() gets
called by dio_complete(). Fix the problem by handling on-disk inode size
update directly in our ->end_io completion handler.

References: https://lore.kernel.org/all/[email protected]
Reported-by: Gao Xiang <[email protected]>
CC: [email protected]
Fixes: 378f32bab371 ("ext4: introduce direct I/O write using iomap infrastructure")
Signed-off-by: Jan Kara <[email protected]>
Tested-by: Joseph Qi <[email protected]>
Reviewed-by: "Ritesh Harjani (IBM)" <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Theodore Ts'o <[email protected]>

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