befs: annotate fs32 on tests for superblock endianness

Does compile-time byteswapping rather than runtime.

Noticed by sparse:
fs/befs/super.c:29:6: warning: cast to restricted __le32
fs/befs/super.c:29:6: warning: cast from restricted fs32
fs/befs/super.c:31:11: warning: cast to restricted __be32
fs/befs/super.c:31:11: warning: cast from restricted fs32
fs/befs/super.c:31:11: warning: cast to restricted __be32
fs/befs/super.c:31:11: warning: cast from restricted fs32
fs/befs/super.c:31:11: warning: cast to restricted __be32
fs/befs/super.c:31:11: warning: cast from restricted fs32
fs/befs/super.c:31:11: warning: cast to restricted __be32
fs/befs/super.c:31:11: warning: cast from restricted fs32
fs/befs/super.c:31:11: warning: cast to restricted __be32
fs/befs/super.c:31:11: warning: cast from restricted fs32
fs/befs/super.c:31:11: warning: cast to restricted __be32
fs/befs/super.c:31:11: warning: cast from restricted fs32
fs/befs/linuxvfs.c:811:7: warning: cast to restricted __le32
fs/befs/linuxvfs.c:811...