uml: fix mconsole stop

Bring back the functionality of stopping user mode linux with the help of
mconsole.

[jdike - the bug being fixed is that the mconsole file descriptor is already
set O_NONBLOCK or not, depending on whether we want no blocking (the normal
case) or we want blocking (when an mconsole stop is in effect), so the
MSG_DONTWAIT is redundant in the normal case, and wrong when we want to
block.]

Signed-off-by: Karol Swietlicki <magotari@gmail.com>
Signed-off-by: Jeff Dike <jdike@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>