btrfs-progs: check: Fix lowmem false alert on tree reloc tree

btrfs-progs: check: Fix lowmem false alert on tree reloc tree

Lowmem mode will report false alert if the fs has tree reloc tree like:
ERROR: shared extent[30011392 4096] lost its parent (parent: 30011392,
level: 1)

The problem is check_shared_block_backref() can't handle tree reloc
tree's self-pointing backref.

And still try to read out the tree block then seeking for the
referencer.

The correct method for it is to check if it's tree reloc root.
In that case, we should check found the ROOT_ITEM of tree reloc tree in
root tree.

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