sheep/md: fix guest file system read-only error

If using md, it is easy to re-produce the error when sheepdog is
doing recovery. The error message is just like as below.

ERROR [io 30895] md_move_object(630) failed to create
                 /mnt/sheepdog/obj2/0082aba600000056
ERROR [io 30895] md_check_and_move(661) move old
                 /mnt/sheepdog/obj/0082aba600000056 to new
                 /mnt/sheepdog/obj2/0082aba600000056 failed

If the above target object is being written unfortunately by other
thread at that time, atomic_create_and_write return -1, and then,
md_exist return false so that guest file system catch it.

Therefore, I think we should check errno if atomic_create_and_write
is failure. If the temp file exist, in another word, someone is
writting it, just return success to avoid the error.

Signed-off-by: Ruoyu <liangry@ucweb.com>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
sheep/request: straightforward error message in sheep_exec_req

Current message as below in sheep_exec_req is rather confused.

ERROR [rw 6471] sheep_exec_req(1133) failed No object found

In fact, I don't know which node sends the request and what it
wants to do. They are the minimum requirements for any investigation.

I think this is very helpful to diagnose an issue.

ERROR [rw 16160] sheep_exec_req(1135) failed No object found,
remote address: 127.0.0.1:7002, op name: GET_HASH

Signed-off-by: Ruoyu <liangry@ucweb.com>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
unittest: fix coredump bug for unit-test

Test case in test_hash.c will call create_vdisks() which need to
check sys->cinfo.flags, but variable 'sys' is uninitialized in
unit-test environment. Therefore we need to init 'sys' before
calling create_vdisks().

Reported-by: Ruoyu <liangry@ucweb.com>
Signed-off-by: Robin Dong <sanbai@taobao.com>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
sheep/md: temp file cannot be removed

d_name is only the file's name without absolute path. To unlink it
successfully, we must give the full name.

Signed-off-by: Ruoyu <liangry@ucweb.com>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>