[PATCH] Add WRITE_ATOMIC_16 support

Tomo, All,

Please find attached a patch that adds support for the new opcode
WRITE_ATOMIC_16 to the bs_rdwr backend.

I try to make it as atomic as possible by using mmap and mlock to ensure that
both the write buffer and the file region we are writing to will be all
present and locked down in memory before I just memcpy() the data.
At that stage it is then assumed that the memcpy() will never fail short.

The patch also adds the three new write atomic fields to the block limits
vpd page and sets some sane defaults.
Maximum amount of write atomic is set to the arbitrary limit of 16MByte
and alignment and granularity is computed based on the blocksize of the
underlying filesystem.

regards
ronnie sahlberg