blktap: a small fix and assorted cleanup

blktap: a small fix and assorted cleanup

The main cleanup item is to convert the index map from a plain
"unsigned long" holding two merged together integers into a proper
structure (at once halving its size on 64-bit).

Additionally, add a previously missing range check of res.id in
blktap_read_ufe_ring() and remove blkif_reqs (which could have been
set via module load option to a value other than the default, which
code elsewhere isn't capable of handling).

Signed-off-by: Jan Beulich <jbeulich@novell.com>