fuse: add missing FR_FORCE

fuse_file_put() was missing the "force" flag for the RELEASE request when
sending synchronously (fuseblk).

If this flag is not set, then a sync request may be interrupted before it
is dequeued by the userspace filesystem.  In this case the OPEN won't be
balanced with a RELEASE.

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Fixes: 5a18ec17 ("fuse: fix hang of single threaded fuseblk filesystem")
Cc: <stable@vger.kernel.org> # v2.6.38+