[BLKTAP]: Fix potential grant entry leaks on error

[BLKTAP]: Fix potential grant entry leaks on error

As it stands grant entries for the actual data are mapped in bulk.
If one of the earlier entries fail to be mapped, it can cause all
subsequent entries to be leaked if they were successfully mapped.

This patch changes it so that we will continue to reap grant entries
even when an error is detected.

There is still a potential leak if create_lookup_pte_addr fails
in fast_flush_area.  This leak is not affected by this patch.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>