Adds a call to RestoreTpl() in DiskIo2ReadWriteDisk(). While the current
implementation does not technically violate spec on raise/restore TPL,
this extra call ensures symmetry between RaiseTpl and RestoreTpl calls,
which makes analysis of TPL correctness simpler and permits certain
non-standard TPL usages that some platforms require.
Signed-off-by: Kenneth Lautner <kenlautner3@gmail.com>