xfs: return from _defer_finish with a clean transaction

The following assertion was seen on generic/051:

XFS: Assertion failed: tp->t_firstblock == NULLFSBLOCK, file: fs/xfs/libxfs5
------------[ cut here ]------------
kernel BUG at fs/xfs/xfs_message.c:102!
invalid opcode: 0000 [#1] SMP PTI
CPU: 2 PID: 20757 Comm: fsstress Not tainted 4.18.0-rc4+ #3969
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.11.1-1 04/01/4
RIP: 0010:assfail+0x23/0x30
Code: c3 66 0f 1f 44 00 00 48 89 f1 41 89 d0 48 c7 c6 88 e0 8c 82 48 89 fa
RSP: 0018:ffff88012dc43c08 EFLAGS: 00010202
RAX: 0000000000000000 RBX: ffff88012dc43ca0 RCX: 0000000000000000
RDX: 00000000ffffffc0 RSI: 000000000000000a RDI: ffffffff828480eb
RBP: ffff88012aa92758 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: f000000000000000 R12: 0000000000000000
R13: ffff88012dc43d48 R14: ffff88013092e7e8 R15: 0000000000000014
FS:  00007f8d689b8e80(0000) GS:ffff88013fd00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f8d689c7000 CR3: 000000012ba6a000 CR4: 00000000000006e0
Call Trace:
 xfs_defer_init+0xff/0x160
 xfs_reflink_remap_extent+0x31b/0xa00
 xfs_reflink_remap_blocks+0xec/0x4a0
 xfs_reflink_remap_range+0x3a1/0x650
 xfs_file_dedupe_range+0x39/0x50
 vfs_dedupe_file_range+0x218/0x260
 do_vfs_ioctl+0x262/0x6a0
 ? __se_sys_newfstat+0x3c/0x60
 ksys_ioctl+0x35/0x60
 __x64_sys_ioctl+0x11/0x20
 do_syscall_64+0x4b/0x190
 entry_SYSCALL_64_after_hwframe+0x49/0xbe

The root cause of the assertion failure is that xfs_defer_finish doesn't
roll the transaction after processing all the deferred items.  Therefore
it returns a dirty transaction to the caller, which leaves the caller at
risk of exceeding the transaction reservation if it logs more items.

Brian Foster's patchset to move the defer_ops firstblock into the
transaction requires t_firstblock == NULLFSBLOCK upon defer_ops
initialization, which is how this was noticed at all.
Reported-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>

fs/xfs/libxfs/xfs_defer.c

@@ -424,6 +424,12 @@ xfs_defer_finish(
 			cleanup_fn(*tp, state, error);
 	}
 
+	/*
+	 * Roll the transaction once more to avoid returning to the caller
+	 * with a dirty transaction.
+	 */
+	if ((*tp)->t_flags & XFS_TRANS_DIRTY)
+		error = xfs_defer_trans_roll(tp, dop);
 out:
 	(*tp)->t_dfops = orig_dop;
 	if (error)