um: Clean up stacktrace dump

[ Upstream commit 273fe1b676cb59d41e177980a981e27806872954 ]

We currently get a few stray newlines, due to the interaction
between printk() and the code here. Remove a few explicit
newline prints to neaten the output.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Richard Weinberger <richard@nod.at>
Stable-dep-of: 0f659ff362ea ("um: Always dump trace for specified task in show_stack")
Signed-off-by: Sasha Levin <sashal@kernel.org>