Fix a missing bounds check in superblock validation.

Note that we don't yet have repair code for this case - repair code for
individual items is generally low priority, since the whole superblock
is checksummed, validated prior to write, and we have backups.
Reported-by: lei lu <llfamsec@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>