fuse: llseek fix race

Fix race between lseek(fd, 0, SEEK_CUR) and read/write.  This was fixed in
generic code by commit 5b6f1eb9

 (vfs: lseek(fd, 0, SEEK_CUR) race condition).

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>