util: make xvalloc() return zeroed memory

This actuall fixes a bug that 045 fails with -ec specified. Instead of add a
memset() to init_erasure_buffer() to fix the bug, I prefer to make xvalloc()
a zeroed memory version, because:

- present CPU and glibc is very good at zero out a memory region.
- zeroed version will avoid future misuage of xvalloc() that expect it to return
  zeroed memory.

Signed-off-by: Liu Yuan <namei.unix@gmail.com>
Reviewed-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>