xen/balloon: remove BUG_ON in increase_reservation

The BUG_ON in increase_reservation is wrong as we have P2M entry
ballooned out page set to balloon scratch page, so it might have a valid
P2M entry at that point.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>