um: Optimize TLB operations v2

Make the code to merge mmap/munmap/mprotect operations in tlb.c
common for userspace and kernel. Kernel tlb operations can now
be merged as well.

Signed-off-by: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Signed-off-by: Richard Weinberger <richard@nod.at>