fuse: set FR_SENT while locked

Otherwise fuse_dev_do_write() could come in and finish off the request, and
the set_bit(FR_SENT, ...) could trigger the WARN_ON(test_bit(FR_SENT, ...))
in request_end().

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Reported-by:  <syzbot+ef054c4d3f64cd7f7cec@syzkaller.appspotmai>
Fixes: 46c34a34 ("fuse: no fc->lock for pqueue parts")
Cc: <stable@vger.kernel.org> # v4.2