fs: tidy up do_sys_openat2() with likely/unlikely

Otherwise gcc 13 generates conditional forward jumps (aka branch
mispredict by default) for build_open_flags() being succesfull.

Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
Link: https://lore.kernel.org/r/20250320092331.1921700-1-mjguzik@gmail.com


Signed-off-by: Christian Brauner <brauner@kernel.org>