Mark fall through cases in switch() statements

Mark fall through cases in switch() statements

Typical covscan complaint, non-empty fall throughs should be marked as
such. There was but a single case which should break instead, namely in
libebt_log.c: It is not critical, since the next case merely asserts
'invert' being zero (which can't be as it was checked before). But while
being at it, introduce log_chk_inv() to consolidate the semantically
equal cases for the various log types.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Florian Westphal <fw@strlen.de>