btrfs: handle invalid root reference found in may_destroy_subvol()

[ Upstream commit 6fbc6f4ac1f4907da4fc674251527e7dc79ffbf6 ]

The may_destroy_subvol() looks up a root by a key, allowing to do an
inexact search when key->offset is -1.  It's never expected to find such
item, as it would break the allowed range of a root id.

Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
(cherry picked from commit ebce7d482d1a08392362ddf936ffdd9244fb1ece)
[Vegard: move changes to ioctl.c due to missing commit
 ec42f167348a1949ac309532aa34760cfc96c92f ("btrfs: Move
 may_destroy_subvol() from ioctl.c to inode.c").]
Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>