fs: generic_block_bmap(): initialize all of the fields in the temp bh

KMSAN (KernelMemorySanitizer, a new error detection tool) reports the
use of uninitialized memory in ext4_update_bh_state():

==================================================================
BUG: KMSAN: use of unitialized memory
CPU: 3 PID: 1 Comm: swapper/0 Tainted: G    B           4.8.0-rc6+ #597
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs
01/01/2011
 0000000000000282 ffff88003cc96f68 ffffffff81f30856 0000003000000008
 ffff88003cc96f78 0000000000000096 ffffffff8169742a ffff88003cc96ff8
 ffffffff812fc1fc 0000000000000008 ffff88003a1980e8 0000000100000000
Call Trace:
 [<     inline     >] __dump_stack lib/dump_stack.c:15
 [<ffffffff81f30856>] dump_stack+0xa6/0xc0 lib/dump_stack.c:51
 [<ffffffff812fc1fc>] kmsan_report+0x1ec/0x300 mm/kmsan/kmsan.c:?
 [<ffffffff812fc33b>] __msan_warning+0x2b/0x40 ??:?
 [<     inline     >] ext4_update_bh_state fs/ext4/inode.c:727
 [<ffffffff8169742a>] _ext4_get_block+0x6ca/0x8a0 fs/ext4/inode.c:759
 [<ffffffff81696d4c>] ext4_get_block+0x8c/0xa0 fs/ext4/inode.c:769
 [<ffffffff814a2d36>] generic_block_bmap+0x246/0x2b0 fs/buffer.c:2991
 [<ffffffff816ca30e>] ext4_bmap+0x5ee/0x660 fs/ext4/inode.c:3177
...
origin description: ----tmp@generic_block_bmap
==================================================================

(the line numbers are relative to 4.8-rc6, but the bug persists
upstream)

The local |tmp| is created in generic_block_bmap() and then passed into
ext4_bmap() => ext4_get_block() => _ext4_get_block() =>
ext4_update_bh_state(). Along the way tmp.b_page is never initialized
before ext4_update_bh_state() checks its value.

[ Use the approach suggested by Kees Cook of initializing the whole bh
  structure.]

Signed-off-by: Alexander Potapenko <glider@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>