btrfs: ctree: Dump the leaf before BUG_ON in btrfs_set_item_key_safe

We have a long standing problem with reversed keys that's detected by
btrfs_set_item_key_safe. This is hard to reproduce so we'd like to
capture more information for later analysis.

Let's dump the leaf content before triggering BUG_ON() so that we can
have some clue on what's going wrong.  The output of tree locks should
help us to debug such problem.

Sample stacktrace:

 generic/522             [00:07:05]
 [26946.113381] run fstests generic/522 at 2019-04-16 00:07:05
 [27161.474720] kernel BUG at fs/btrfs/ctree.c:3192!
 [27161.475923] invalid opcode: 0000 [#1] PREEMPT SMP
 [27161.477167] CPU: 0 PID: 15676 Comm: fsx Tainted: G        W         5.1.0-rc5-default+ #562
 [27161.478932] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.12.0-0-ga698c89-prebuilt.qemu.org 04/01/2014
 [27161.481099] RIP: 0010:btrfs_set_item_key_safe+0x146/0x1c0 [btrfs]
 [27161.485369] RSP: 0018:ffffb087499e39b0 EFLAGS: 00010286
 [27161.486464] ...