ext4: dio take shared inode lock when overwriting preallocated blocks

In the dio write path, we only take shared inode lock for the case of
aligned overwriting initialized blocks inside EOF. But for overwriting
preallocated blocks, it may only need to split unwritten extents, this
procedure has been protected under i_data_sem lock, it's safe to
release the exclusive inode lock and take shared inode lock.

This could give a significant speed up for multi-threaded writes. Test
on Intel Xeon Gold 6140 and nvme SSD with below fio parameters.

 direct=1
 ioengine=libaio
 iodepth=10
 numjobs=10
 runtime=60
 rw=randwrite
 size=100G

And the test result are:
Before:
 bs=4k       IOPS=11.1k, BW=43.2MiB/s
 bs=16k      IOPS=11.1k, BW=173MiB/s
 bs=64k      IOPS=11.2k, BW=697MiB/s

After:
 bs=4k       IOPS=41.4k, BW=162MiB/s
 bs=16k      IOPS=41.3k, BW=646MiB/s
 bs=64k      IOPS=13.5k, BW=843MiB/s

Signed-off-by: Zhang Yi <[email protected]>
Reviewed-by: Jan Kara <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Theodore Ts'o <[email protected]>

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