udf: Fix off-by-one error when discarding preallocation

udf: Fix off-by-one error when discarding preallocation

[ Upstream commit 490a849b4a3d6e9bd67f9aa4385502dadbb70a32 ]

The condition determining whether the preallocation can be used had
an off-by-one error so we didn't discard preallocation when new
allocation was just following it. This can then confuse code in
inode_getblk().

CC: stable@vger.kernel.org
Fixes: 189b02ac597c ("udf: Discard preallocation before extending file with a hole")
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Sasha Levin <sashal@kernel.org>