Make some small improvements for arch/um/kernel/um_arch.c.
Signed-off-by: WANG Cong <xiyou.wangcong@gmail.com> Acked-by: Jeff Dike <jdike@addtoit.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>