ebtables: check struct type before computing gap

Check struct type before dereferencing fields in ebt_entry.
Failure to check can cause oops.

Signed-off-by: Chuck Ebbert <76306.1226@compuserve.com>
Acked-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Adrian Bunk <bunk@stusta.de>