sbi->s_group_desc is an array of pointers to buffer_head.  memcpy() of
buffer size from address of buffer_head is a bad idea - it will generate
junk in any case, may oops if buffer_head is close to the end of slab
page and next page is not mapped and isn't what was intended there.
IOW, ->b_data is missing in that call.  Fortunately, result doesn't go
into the primary on-disk data structures, so only backup ones get crap
written to them; that had allowed this bug to remain unnoticed until
now.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>