DESC
UML - SKAS: avoid leak of kernel memory by SKAS patch.
EDESC
Part 1: fix the kernel leak.
init_new_context was called, and then __init_new_context; they both clear
the LDT (by setting its size to 0) and alloc a new one; and since the LDT size
has been cleared, alloc_ldt does not free() the LDT. It it exposed only if actually
the UML process has an LDT to allocate, i.e. if the UML kernel thread had an LDT
on the host when forking the init process.

Part 2: avoid allocating a useless LDT for the init inside UML.
Until now, when opening /proc/mm the SKAS patch copied the current LDT
like on fork().
Since currently UML does not expect the userspace thread to inherit
its own LDT, we can avoid this and save 4k for each child (useful when hosting
lots of UML). And besides, that was waste memory.

Actually UML never sets its one LDT, since it does not use
TLS; it can just inherits it from his fathers, but it's useless.