btrfs: send: remove WARN_ON for readonly mount

We log warning if root::orphan_cleanup_state is not set to
ORPHAN_CLEANUP_DONE in btrfs_ioctl_send(). However if the filesystem is
mounted as readonly we skip the orphan item cleanup during the lookup
and root::orphan_cleanup_state remains at the init state 0 instead of
ORPHAN_CLEANUP_DONE (2). So during send in btrfs_ioctl_send() we hit the
warning as below.

  WARN_ON(send_root->orphan_cleanup_state != ORPHAN_CLEANUP_DONE);

WARNING: CPU: 0 PID: 2616 at /Volumes/ws/btrfs-devel/fs/btrfs/send.c:7090 btrfs_ioctl_send+0xb2f/0x18c0 [btrfs]
::
RIP: 0010:btrfs_ioctl_send+0xb2f/0x18c0 [btrfs]
::
Call Trace:
::
_btrfs_ioctl_send+0x7b/0x110 [btrfs]
btrfs_ioctl+0x150a/0x2b00 [btrfs]
::
do_vfs_ioctl+0xa9/0x620
? __fget+0xac/0xe0
ksys_ioctl+0x60/0x90
__x64_sys_ioctl+0x16/0x20
do_syscall_64+0x49/0x130
entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reproducer:
  mkfs.btrfs -fq /dev/sdb
  mount /dev/sdb /btrfs
  btrfs subvolume create /btrfs/sv1
  btrfs subvolume snapshot -r /btrfs/sv1 /btrfs/ss1
  umount /btrfs
  mount -o ro /dev/sdb /btrfs
  btrfs send /btrfs/ss1 -f /tmp/f

The warning exists because having orphan inodes could confuse send and
cause it to fail or produce incorrect streams.  The two cases that would
cause such send failures, which are already fixed are:

1) Inodes that were unlinked - these are orphanized and remain with a
   link count of 0. These caused send operations to fail because it
   expected to always find at least one path for an inode. However this
   is no longer a problem since send is now able to deal with such
   inodes since commit 46b2f459 ("Btrfs: fix send failure when root
   has deleted files still open") and treats them as having been
   completely removed (the state after an orphan cleanup is performed).

2) Inodes that were in the process of being truncated. These resulted in
   send not knowing about the truncation and potentially issue write
   operations full of zeroes for the range from the new file size to the
   old file size. This is no longer a problem because we no longer
   create orphan items for truncation since commit f7e9e8fc ("Btrfs:
   stop creating orphan items for truncate").

As such before these commits, the WARN_ON here provided a clue in case
something went wrong. Instead of being a warning against the
root::orphan_cleanup_state value, it could have been more accurate by
checking if there were actually any orphan items, and then issue a
warning only if any exists, but that would be more expensive to check.
Since orphanized inodes no longer cause problems for send, just remove
the warning.

Reported-by: Christoph Anton Mitterer <calestyo@scientia.net>
Link: https://lore.kernel.org/linux-btrfs/21cb5e8d059f6e1496a903fa7bfc0a297e2f5370.camel@scientia.net/


CC: stable@vger.kernel.org # 4.19+
Suggested-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>