blk: update checksum calculation algorithm

- Update the checksum handling to handle BE systems.
- Improve the way the chk-sums are calculated.
- Adjust the checksum unittest to be endianess aware.

Initial version by @mdalecki
Merge pull request #57 from tomaszkapela/pmemblk_checksum_update

blk: update checksum calculation algorithm
vmem: fix possible attempt to unlock a mutex that is not locked
Merge pull request #58 from krzycz/vmem-ctl_mtx_unlock

vmem: fix possible attempt to unlock a mutex that is not locked