Revision 224465

MFC r216754:
  Make EBR probe method less strictly to be able detect EBRs with
  small non fatal inconsistency. EBR may contain boot loader and sometimes
  it just has some garbage data. Now this does not prevent FreeBSD to use
  extended partitions. But since we do not support bootcode for EBR we mark
  tables which have non empty boot area as corrupt. This does make them
  readonly and we can not damage this data.

MFC r223594:
  EBR could contain an early stage of boot code. But we do not support it.
  Remove message about non empty bootcode, we can not break something
  while GEOM_PART_EBR_COMPAT is defined.

  But without GEOM_PART_EBR_COMPAT any changes in EBR are allowed and we
  can accidentally wipe the boot code. To do not break anything save
  the first EBR chunk and keep it untouched each time when we are
  changing EBR. Note that we are still not support boot code for EBR.

  PR:		kern/141235