udf: Release preallocation on last writeable close

Commit 6fb1ca92 "udf: Fix race between write(2) and close(2)"
changed the condition when preallocation is released. The idea was that
we don't want to release the preallocation for an inode on close when
there are other writeable file descriptors for the inode. However the
condition was written in the opposite way so we released preallocation
only if there were other writeable file descriptors. Fix the problem by
changing the condition properly.

CC: stable@vger.kernel.org
Fixes: 6fb1ca92


Reported-by: Fabian Frederick <fabf@skynet.be>
Signed-off-by: Jan Kara <jack@suse.cz>