fhandle: do_handle_open() should get FD with user flags

fhandle: do_handle_open() should get FD with user flags

In f07c7cc4684a, do_handle_open() was switched to use the automatic
cleanup method for getting a FD. In that change it was also switched
to pass O_CLOEXEC unconditionally to get_unused_fd_flags() instead
of passing the user-specified flags.

I don't see anything in that commit description that indicates this was
intentional, so I am assuming it was an oversight.

With this fix, the FD will again be opened with, or without, O_CLOEXEC
according to what the user requested.

Fixes: f07c7cc4684a ("fhandle: simplify error handling")
Signed-off-by: Thomas Bertschinger <tahbertschinger@gmail.com>
Link: https://lore.kernel.org/20250814235431.995876-4-tahbertschinger@gmail.com
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>