um: Implement probe_kernel_read()

commit f75b1b1bedfb498cc43a992ce4d7ed8df3b1e770 upstream.

UML needs it's own probe_kernel_read() to handle kernel
mode faults correctly.
The implementation uses mincore() on the host side to detect
whether a page is owned by the UML kernel process.

This fixes also a possible crash when sysrq-t is used.
Starting with 3.10 sysrq-t calls probe_kernel_read() to
read details from the kernel workers. As kernel worker are
completely async pointers may turn NULL while reading them.

Signed-off-by: Richard Weinberger <richard@nod.at>
Cc: <stian@nixia.no>
Cc: <tj@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>