uml: sysrq and mconsole fixes

Fix the passing of printk output back to the mconsole client.  The existing
code was somewhat confused, accumulating output in a buffer, but writing it
out entirely whenever a new chunk was added.  This is fixed.

The earlier include cleanups caused linux/sysrq.h to not be included - this is
fixed by adding the include back, under CONFIG_MAGIC_SYSRQ.

CONFIG_MAGIC_SYSRQ is also defaulted to on in defconfig.

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>
diff --git a/arch/um/defconfig b/arch/um/defconfig
index ed3196d..658f654 100644
--- a/arch/um/defconfig
+++ b/arch/um/defconfig
@@ -80,7 +80,7 @@
 # CONFIG_HOSTFS is not set
 # CONFIG_HPPFS is not set
 CONFIG_MCONSOLE=y
-# CONFIG_MAGIC_SYSRQ is not set
+CONFIG_MAGIC_SYSRQ=y
 CONFIG_NEST_LEVEL=0
 # CONFIG_HIGHMEM is not set
 CONFIG_KERNEL_STACK_ORDER=0