For snapshot shared tree blocks with source subvolume, the keyed backref
counter only counts the exclusive owned references.
In the following case, 258 is a snapshot of 257, which inherits all the
reference to this data extent.
------
item 4 key (
12582912 EXTENT_ITEM 524288) itemoff 3741 itemsize 140
refs 179 gen 9 flags DATA
extent data backref root 257 objectid 258 offset 0 count 49
extent data backref root 257 objectid 257 offset 0 count 1
extent data backref root 256 objectid 258 offset 0 count 128
extent data backref root 256 objectid 257 offset 0 count 1
------
However lowmem mode used to iterate the whole inode to find all
references, and doesn't care if a reference is already counted by the
shared tree block.
Add the test case to check it.
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>