ovl: narrow locking in ovl_whiteout()

ovl_whiteout() relies on the workdir i_rwsem to provide exclusive access
to ofs->whiteout which it manipulates.  Rather than depending on this,
add a new mutex, "whiteout_lock" to explicitly provide the required
locking.  Use guard(mutex) for this so that we can return without
needing to explicitly unlock.

Then take the lock on workdir only when needed - to lookup the temp name
and to do the whiteout or link.

Signed-off-by: NeilBrown <neil@brown.name>
Link: https://lore.kernel.org/20250716004725.1206467-19-neil@brown.name
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>

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