fuse: don't increment nlink in link()

The fuse_iget() call in create_new_entry() already updated the inode with
all the new attributes and incremented the attribute version.

Incrementing the nlink will result in the wrong count.  This wasn't noticed
because the attributes were invalidated right after this.

Updating ctime is still needed for the writeback case when the ctime is not
refreshed.

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>