ibmasm: Replace kzalloc() + copy_from_user() with memdup_user_nul()

Replace kzalloc() followed by copy_from_user() with memdup_user_nul() to
improve and simplify remote_settings_file_write().

No functional changes intended.

Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Link: https://lore.kernel.org/r/20250905103247.423840-2-thorsten.blum@linux.dev


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>