udf: Remove pointless union in udf_inode_info

[ Upstream commit 382a2287bf9cd283206764572f66ab12657218aa ]

We use only a single member out of the i_ext union in udf_inode_info.
Just remove the pointless union.

Signed-off-by: Jan Kara <jack@suse.cz>
Stable-dep-of: fc8033a34a3c ("udf: Preserve link count of system files")
Signed-off-by: Sasha Levin <sashal@kernel.org>