btrfs: inode: Verify inode mode to avoid NULL pointer dereference

      [BUG]
When accessing a file on a crafted image, btrfs can crash in block layer:

  BUG: unable to handle kernel NULL pointer dereference at 0000000000000008
  PGD 136501067 P4D 136501067 PUD 124519067 PMD 0
  CPU: 3 PID: 0 Comm: swapper/3 Not tainted 5.0.0-rc8-default #252
  RIP: 0010:end_bio_extent_readpage+0x144/0x700
  Call Trace:
   <IRQ>
   blk_update_request+0x8f/0x350
   blk_mq_end_request+0x1a/0x120
   blk_done_softirq+0x99/0xc0
   __do_softirq+0xc7/0x467
   irq_exit+0xd1/0xe0
   call_function_single_interrupt+0xf/0x20
   </IRQ>
  RIP: 0010:default_idle+0x1e/0x170

[CAUSE]
The crafted image has a tricky corruption, the INODE_ITEM has a
different type against its parent dir:

        item 20 key (268 INODE_ITEM 0) itemoff 2808 itemsize 160
                generation 13 transid 13 size 1048576 nbytes 1048576
                block group 0 mode 121644 links 1 uid 0 gid 0 rdev 0
                sequence 9 flags 0x0(no