fstests: add some missing _require_loop's

Got a new box running overnight fstests and noticed a couple of failures
because I forgot to enable loop device support.  Fix these two tests to
have _require_loop so they don't fail if there's no loop device support.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Zorro Lang <zlang@kernel.org>

tests/btrfs/012

diff --git a/tests/btrfs/012 b/tests/btrfs/012
index 29552b1..60461a3 100755
--- a/tests/btrfs/012
+++ b/tests/btrfs/012
@@ -30,6 +30,7 @@
 _require_command "$E2FSCK_PROG" e2fsck
 # ext4 does not support zoned block device
 _require_non_zoned_device "${SCRATCH_DEV}"
+_require_loop
 
 BLOCK_SIZE=`_get_block_size $TEST_DIR`
 

tests/generic/648

diff --git a/tests/generic/648 b/tests/generic/648
index e5c743c..d7bf569 100755
--- a/tests/generic/648
+++ b/tests/generic/648
@@ -39,6 +39,7 @@
 _require_cp_reflink
 _require_dm_target error
 _require_command "$KILLALL_PROG" "killall"
+_require_loop
 
 echo "Silence is golden."