client: propagate flush errors to fclose/fsync

client: propagate flush errors to fclose/fsync

Ensure that all calls to _flush get an explicit
context (rather than implicit PutInode used
before), which tags any error onto the Inode object.

Later, read the error from the Inode object in _release_fh
and ensure all callers of _release_fh properly handle
its return code.

Signed-off-by: John Spray <john.spray@redhat.com>