zfs: Fix malloc() success check

This code was hitting the error code path whenever malloc() succeeded
rather than when it failed, so presumably this part of the code hasn't
been tested. I had to apply this fix (and others) to get U-Boot to boot
from ZFS on an Nvidia Jetson TX2 NX SoM (an aarch64 computer).

Signed-off-by: Phaedrus Leeds <mwleeds@mailtundra.com>
diff --git a/fs/zfs/zfs.c b/fs/zfs/zfs.c
index 1fec96c..14779de 100644
--- a/fs/zfs/zfs.c
+++ b/fs/zfs/zfs.c
@@ -655,7 +655,7 @@
 											dn->endian)
 				<< SPA_MINBLOCKSHIFT;
 			*buf = malloc(size);
-			if (*buf) {
+			if (!*buf) {
 				err = ZFS_ERR_OUT_OF_MEMORY;
 				break;
 			}