Revert "zfsinfo: Correct a check for error allocating memory"

Revert "zfsinfo: Correct a check for error allocating memory"

Original commit is wrong because grub_file_get_device_name() may return NULL
if we use implicit $root. Additionally, the grub_errno is guaranteed to be
GRUB_ERR_NONE at the beginning of a command. So, everything should work as
expected and Coverity report, CID 73668, WRT to this code should be treated
as false positive.

This reverts commit 7aab03418 (zfsinfo: Correct a check for error allocating memory).

Fixes: 7aab03418 (zfsinfo: Correct a check for error allocating memory)
Signed-off-by: Vladimir Serbinenko <phcoder@gmail.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>