Add checks for device names in dmsetup and show proper error messages.

Add checks for device names in dmsetup and show proper error messages.

Checks added for DM device names to allow only names < DM_NAME_LEN,
otherwise a part of lengthy name would be silently ignored and could
cause confusion while using dmsetup. Also, the name should not contain
'/' character, if it is used in context of creating a new device
or renaming the existing one (because we do not consider full path
to devices, they do not exist in filesystem yet) and appropriate error
messages are shown.