reiserfs: add check for an invalid ih_entry_count

commit d24396c5290ba8ab04ba505176874c4e04a2d53c upstream.

when directory item has an invalid value set for ih_entry_count it might
trigger use-after-free or out-of-bounds read in bin_search_in_dir_item()

ih_entry_count * IH_SIZE for directory item should not be larger than
ih_item_len

Link: https://lore.kernel.org/r/20201101140958.3650143-1-rkovhaev@gmail.com


Reported-and-tested-by:  <syzbot+83b6f7cf9922cae5c4d7@syzkaller.appspotmail.com>
Link: https://syzkaller.appspot.com/bug?extid=83b6f7cf9922cae5c4d7


Signed-off-by: Rustam Kovhaev <rkovhaev@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>