um: header debriding - mem_user.h

get_vm(), add_iomem(), phys_offset() dead since 2004;
init_mem_user() and setup_memory() - since before the initial merge.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Richard Weinberger <richard@nod.at>