arch/um/os-Linux/sys-i386/task_size.c: improve a bit

Improve this code a bit: check sigaction's return value and remove a useless
fflush().

Acked-by: Jeff Dike <jdike@addtoit.com>
Signed-off-by: WANG Cong <wangcong@zeuux.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>