fs: Fix filename init after recent refactoring

fs: Fix filename init after recent refactoring

getname_flags() should save __user pointer "filename" in filename->uptr.
However, this logic is broken by a recent refactoring. Fix it by passing
__user pointer filename to helper initname().

Fixes: 611851010c74 ("fs: dedup handling of struct filename init and refcounts bumps")
Cc: Mateusz Guzik <mjguzik@gmail.com>
Cc: Christian Brauner <brauner@kernel.org>
Signed-off-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/20250409220534.3635801-1-song@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>