um: Hard-code the result of 'uname -s'

We rely on 'uname -s' returning 'Linux' because there are os-Linux/
directories, but no other os-*/.

Supporting a non-Linux host is unlikely to happen.

Let's hard-code 'Linux'.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Richard Weinberger <richard@nod.at>