fs: move file_start_write() into direct_splice_actor()

The callers of do_splice_direct() hold file_start_write() on the output
file.

This may cause file permission hooks to be called indirectly on an
overlayfs lower layer, which is on the same filesystem of the output
file and could lead to deadlock with fanotify permission events.

To fix this potential deadlock, move file_start_write() from the callers
into the direct_splice_actor(), so file_start_write() will not be held
while splicing from the input file.

Suggested-by: Josef Bacik <[email protected]>
Link: https://lore.kernel.org/r/20231128214258.GA2398475@perftesting/
Reviewed-by: Jan Kara <[email protected]>
Signed-off-by: Amir Goldstein <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Christian Brauner <[email protected]>

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