fusefs: don't force direct io for files opened O_WRONLY

fusefs: don't force direct io for files opened O_WRONLY

Previously fusefs would treat any file opened O_WRONLY as though the
FOPEN_DIRECT_IO flag were set, in an attempt to avoid issuing reads as part
of a RMW write operation on a cached part of the file.  However, the FUSE
protocol explicitly allows reads of write-only files for precisely that
reason.

Sponsored by:	The FreeBSD Foundation