hammer2 - Finally fix the 'file winds up with some zeros' bug
* Fix this bug, which has been reported several times this year
but until nikita's hands-on I couldn't find it.
* The bug occurs when H2 must transition from the 512 bytes of
file data directly embedded in the inode to a block table as
a file grows past 512 bytes.
Basically what happens is that the buffer cache buffer holding
the data in-transition can get lost.
The result is that the first write() that overlaps seek offset
512 in a small file can sometimes cause the prior data at the
start of the file to be lost.
* To solve the problem the buffer is now held across the transition
and explicitly dirtied to force it to be flushed to the now-
realized block table.
Reported-by: nikita and several others this year
UnifiedSplitRaw
