Description: btrfs: fix NULL pointer dereference when deleting device by invalid id
References:
 https://www.openwall.com/lists/oss-security/2021/08/25/3
 https://lore.kernel.org/linux-btrfs/CAFcO6XO5TC5sEo-C9JGC75JkNAzkOSSLA3a=bwQqXFFbRTZ7Gw@mail.gmail.com/T/#md4b850f33616b7364f86e6fed144abc925f3669c
 https://lore.kernel.org/linux-btrfs/20210806102415.304717-1-wqu@suse.com/T/#u
 https://bugzilla.redhat.com/show_bug.cgi?id=1997958
Notes:
 carnil> Commit fixes a27a94c2b0c7 ("btrfs: Make
 carnil> btrfs_find_device_by_devspec return btrfs_device directly") in
 carnil> 4.20-rc1.
Bugs:
upstream: released (5.15-rc1) [e4571b8c5e9ffa1e85c0c671995bd4dcc5c75091]
5.10-upstream-stable: released (5.10.62) [c43add24dffdbac269d5610465ced70cfc1bad9e]
4.19-upstream-stable: N/A "Vulnerable code introduced later"
4.9-upstream-stable: N/A "Vulnerable code introduced later"
sid: released (5.14.6-1)
5.10-bullseye-security: released (5.10.46-5) [bugfix/all/btrfs-fix-NULL-pointer-dereference-when-deleting-dev.patch] 
4.19-buster-security: N/A "Vulnerable code introduced later"
4.9-stretch-security: N/A "Vulnerable code introduced later"