Grant unmap error checking in Dom0

[Zoltan Kiss <zoltan.kiss@citrix.com>]

Hi,

Just out of curiosity I've checked how Dom0 handles errors during grant 
unmapping. Usually there is a BUG_ON(ret) for the return value of 
gnttab_unmap_refs in blkback and netback, gntdev drops just a WARN.
The return value can be non-zero only if Xen failed to copy the map 
operations back and forth to the guest supplied memory, so it's 
reasonable to crash there. However I'm wondering why gntdev is happy 
with just a WARN.
Another thing, we don't check the status of the operations if the return 
value is zero. We shouldn't normally do that, Xen logs info messages in 
some cases, but not always (e.g. XSM or IOMMU problems). For debugging 
purposes however it could be useful to have the ability to turn on 
checking in Dom0. A quick and dirty way to do this is to use 
printk_get_level to figure out if the loglevel is e.g. KERN_NOTICE or 
lower, but I'm sure there is a better way to do this :) It would be an 
overkill to introduce new config option, I'm thinking a runtime 
parameter to check in an unlikely(), so it won't cause performance 
penalty for normal operation. Any opinions on that?

Zoli