arch/um/kernel/mem.c: fix a shadowed variable

Fix a shadowed variable in arch/um/kernel/mem.c, since there is a global
variable has the same name.

Cc: Jeff Dike <jdike@linux.intel.com>
Signed-off-by: WANG Cong <xiyou.wangcong@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>