e2fsck: sanity check the journal inode number

e2fsck: sanity check the journal inode number

E2fsck replays the journal before sanity checking the full superblock.
So it's possible that the journal inode number is not valid relative
to the number of block groups.  So to avoid potentially an array
bounds overrun, sanity check this before trying to find the journal
inode.

Reported-by: Nils Bars <nils.bars@rub.de>
Reported-by: Moritz Schlögel <moritz.schloegel@rub.de>
Reported-by: Nico Schiller <nico.schiller@rub.de>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>