Fix asan error when copying super block to backup super blocks

Address santizier caught a bug where we memcpy a full block size into
the backup super block, but the source may be offset 1024 bytes into
a block size allocation.  The destination buffer is zeroed by calloc,
so just copy the super block size instead.

Change-Id: I87af8e3349a63940905697466f62532c50fab720
1 file changed