btrfs-progs: lowmem check: Fix false alerts for image with shared block ref only...

btrfs-progs: lowmem check: Fix false alerts for image with shared block ref only backref

[BUG]
For image with shared block ref only metadata item like:
------
        item 66 key (21573632 METADATA_ITEM 0) itemoff 3971 itemsize 24
                refs 66 gen 9 flags TREE_BLOCK|FULL_BACKREF
                tree block skinny level 0
        item 0 key (21573632 SHARED_BLOCK_REF 21676032) itemoff 3995 itemsize 0
                shared block backref
        item 1 key (21573632 SHARED_BLOCK_REF 21921792) itemoff 3995 itemsize 0
                shared block backref
        item 2 key (21573632 SHARED_BLOCK_REF 21995520) itemoff 3995 itemsize 0
                shared block backref
        item 3 key (21573632 SHARED_BLOCK_REF 22077440) itemoff 3995 itemsize 0
                shared block backref
...
------

Lowmem mode check will report false alerts like:
------
ERROR: extent[21573632 4096] backref lost (owner: 256, level: 0)
------

[CAUSE]
In fact, the false alerts are not even from extent tree verfication,  but
a fs tree helper which is designed to make sure there is some tree block
referring to the fs tree block.

The idea is to find inlined tree backref then keyed TREE_BLOCK_REF_KEY.
However it missed SHARED_BLOCK_REF_KEY, and caused such false alert.

[FIX]
Add SHARED_BLOCK_REF_KEY to make the warning shut up.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>