Fix "constant 2435016766 overflows int32" build failure on arm
Fix "constant 2435016766 overflows int32" build failure on arm

Cast to uint32 to fix the int32 overflow, and define BTRFS_SUPER_MAGIC
locally to fix the next failure on darwin.

rfjakob/gocryptfs,rfjakob/gocryptfs,rfjakob/gocryptfs,rfjakob/gocryptfs