Support improperly formated device numbers

Support improperly formated device numbers

There are kernel drivers (smblk) which set '-1' as their device major number.
This number is listed in /proc/devices then - but the kernel itself is using
just 12 bits - thus device is accessible via 4095 - there is posted patch
for 3.4 to fix this behavior (0 for auto allocation was mean to be used).

However to still allow using such devices with older kernels add some code
to use same behavior - so cut 12 bits from the major number from /proc/devices.

For now use log_warn() - maybe the severity of the message could be lowered
to just verbose level.