When a file is opened write-only and a partial block was written,
buffered I/O would try and read the whole block in. This would
result in a hung thread, since there was no open (fuse filehandle)
that allowed reading. This patch avoids the problem by forcing
DIRECT_IO for this case.
It also sets DIRECT_IO when the file system specifies the FN_DIRECTIO
flag in its reply to the open.
Tested by: nishida@asusa.net, freebsd@moosefs.com
PR: 194293, 206238
MFC after: 2 weeks
8d3f87b2b7