ext4: fix EXT4_MAX_LOGICAL_BLOCK macro

ext4 supports max number of logical blocks in a file to be 0xffffffff.
(This is since ext4_extent's ee_block is __le32).
This means that EXT4_MAX_LOGICAL_BLOCK should be 0xfffffffe (starting
from 0 logical offset). This patch fixes this.

The issue was seen when ext4 moved to iomap_fiemap API and when
overlayfs was mounted on top of ext4. Since overlayfs was missing
filemap_check_ranges(), so it could pass a arbitrary huge length which
lead to overflow of map.m_len logic.

This patch fixes that.

Fixes: d3b6f23f ("ext4: move ext4_fiemap to use iomap framework")
Reported-by: syzbot+77fa5bdb65cc39711820@syzkaller.appspotmail.com
Signed-off-by: NRitesh Harjani <riteshh@linux.ibm.com>
Reviewed-by: NJan Kara <jack@suse.cz>
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20200505154324.3226743-2-hch@lst.deSigned-off-by: NTheodore Ts'o <tytso@mit.edu>

fs/ext4/ext4.h

@@ -733,7 +733,7 @@ enum {
 #define EXT4_MAX_BLOCK_FILE_PHYS	0xFFFFFFFF
 
 /* Max logical block we can support */
-#define EXT4_MAX_LOGICAL_BLOCK		0xFFFFFFFF
+#define EXT4_MAX_LOGICAL_BLOCK		0xFFFFFFFE
 
 /*
  * Structure of an inode on the disk