generic/37[3-4]: use bind mount instead of 2nd _scratch_mount

generic/373 and generic/374 test that cross-mountpoint reflink/dedupe
fail with EXDEV.

For this test, _scratch_mount is called a 2nd time with same mount
arguments, but a different value of SCRATCH_MNT.

This practice is incorrect for overlayfs, which should not be using
the same upper/work dirs on two different overlay super blocks.

Change the test to use bind mount instead, which results in the
expected setup for both block device mount and overlay mount.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>

tests/generic/373

@@ -64,7 +64,7 @@ blksz=65536
 sz=$((blksz * blocks))
 
 echo "Mount otherdir"
-SCRATCH_MNT=$otherdir _scratch_mount
+$MOUNT_PROG --bind $SCRATCH_MNT $otherdir
 
 echo "Create file"
 _pwrite_byte 0x61 0 $sz $testdir/file >> $seqres.full
@@ -82,7 +82,7 @@ md5sum $testdir/file | _filter_scratch
 test -e $othertestdir/otherfile && echo "otherfile should not exist"
 
 echo "Unmount otherdir"
-SCRATCH_MNT=$otherdir _scratch_unmount
+$UMOUNT_PROG $otherdir
 rm -rf $otherdir
 
 # success, all done

tests/generic/374

@@ -63,7 +63,7 @@ blksz=65536
 sz=$((blocks * blksz))
 
 echo "Mount otherdir"
-SCRATCH_MNT=$otherdir _scratch_mount
+$MOUNT_PROG --bind $SCRATCH_MNT $otherdir
 
 echo "Create file"
 _pwrite_byte 0x61 0 $sz $testdir/file >> $seqres.full
@@ -80,7 +80,7 @@ echo "Check output"
 md5sum $testdir/file $othertestdir/otherfile | filter_md5
 
 echo "Unmount otherdir"
-SCRATCH_MNT=$otherdir _scratch_unmount
+$UMOUNT_PROG $otherdir
 rm -rf $otherdir
 
 # success, all done