reiserfs: fix kernel panic on corrupted directory

When reading corrupted reiserfs directory data, d_reclen could be a
negative number or a big positive number, this can lead to kernel panic or
oop.  The following patch adds a sanity check.

Signed-off-by: Lepton Wu <ytht.net@gmail.com>
Cc: Chris Mason <chris.mason@oracle.com>
Cc: Jeff Mahoney <jeffm@suse.com>
Cc: "Vladimir V. Saveliev" <vs@namesys.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>