um: Fix len of file in create_pid_file

sizeof gives us the size of the pointer variable, not of the
area it points to. So the number of bytes copied by umid_file_name()
is 8.
We should pass in the correct length of the file buffer.

Signed-off-by: Wen Yang <wenyang@linux.alibaba.com>
Signed-off-by: Richard Weinberger <richard@nod.at>