Fix buffer overrun issue of lld name

The string buffer of lld name in tgtadm_req struct (req->lld) will not
be null-terminated if user specifies very long lld name for the
argument of -L or --lld option.

This is because the lld name is copied with strncpy function and its
size argument is the same as buffer size.  In such a case, strncpy()
can truncate the string without appending a terminating null byte.

As a result, accesses to the lld name in mtask_execute function, for
instance, strlen(req->lld) or eprintf("...%s\n", req->lld), can
overrun.

This patch fixes the issue by setting a terminating null byte at the
end of the lld name buffer before mtask_execute() uses it.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>