tests/func/058: fix supurious failure

Reviewed-by: Robin Dong <sanbai@taobao.com>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
lib: change xmalloc() to xvalloc() in sd_inode.c

The default_read_from_path() returned "Invalid argument" when we running
sheepdog cluster in test environment with sheep option "-D". The reason
is that the allocating memory for reading and writing btree-object is not
aligned. Therefore we should use xvalloc() instead of xmalloc() to allocate
aligned memory.

Signed-off-by: Robin Dong <sanbai@taobao.com>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
sheep: go to panic if using unaligned memory in direct-io mode

When startup sheep with option "-D", the plain_store will check alignment of
read/write buffer, but if the buffer is not aligned, it will do nothing if
it dose not choose "--enable-debug" for configuration. Then sheep will open
file with "O_DIRECT" and call pread() with unaligned buffer, it will cause
error io and remove the disk from md which is totally wrong behavior.
So, we should make the check strong enough: sheep will panic directlly if
finding unaligned buffer.

Signed-off-by: Robin Dong <sanbai@taobao.com>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>